The physiology of reproduction

Front matter

tinh At eee eae Pee ee “ ewe irae lator teat Sarat

Se wh Petit Uti te ne cerca pci a ad ae teh crisciene Eixaacaipineconis oe oe ieneent salah a aytaeal dis eae a peat edad aed (i ae cla hen se beipry int am

Dani te ‘far? at * petiate ta veda wise, ia

Preface

TuIs is the first time that the Physiology of the Organs of Reproduction has been presented in a complete form, and many who desire to obtain more precise knowledge regarding the problems with which it deals, than is to be found in text-books of Physiology, will welcome the appearance of Dr. Marshall’s book. The importance of such knowledge to the community in general is now becoming recognised, and the interest which the subject awakens is no longer confined to members of the medical profession and to breeders of animals. Especially will the work furnish a much needed introduction to the science of Eugenics, whilst the multiplicity of facts which are set forth, and the manner in which questions of difficulty are discussed, will have the effect at once of satisfying and of stimulating inquiry in a most important, if hitherto somewhat neglected, branch of Physiology.

E. A. SCHAFER.

UNIVERSITY, EDINBURGH, June 1910.

vii

Contents

INTRODUCTION . . =i CHAPTER I THE BREEDING SEASON. . e . ‘ r . “ . 4 Protozoa—Ccelenterata—Nemertea, &c.—Annelida—Arthropoda — Mollusca — Echinodermata — Cephalochordata — Pisces— Amphibia — Reptilia — Aves — Mammalia—Periodicity of Breeding, &c.

CHAPTER II THE CESTROUS CYCLE IN THE MAMMALIA ‘ 2 ‘ : - 35 Monotremata—Marsupialia—Rodentia—Ungulata—Cetacea— Carnivora—lInsectivora—Cheiroptera—Primates. CHAPTER III THE CHANGES THAT OCCUR IN THE NON-PREGNANT UTERUS DURING THE ESTROUS CYCLE . F ‘ z 2 7 » 75

The Cycle in Man—Monkeys—Lemurs—lInsectivora—Carnivores —Ungulates.

Introduction

Srnce the time when physiology first became an organised science many volumes have been written on the digestive, excretory, nervous, and other systems of the body, but no attempt has yet been made to supply those interested in the reproductive processes with a comprehensive treatise dealing with this branch of knowledge. Indeed, in most text-books on physiology now commonly in use either the section devoted to the reproductive organs is restricted to a few final pages seldom free from error, or else the subject is entirely omitted. Yet generative physiology forms the basis of gynecological science, and must ever bear a close relation to the study of animal breeding. In writing the present volume, therefore, I have been actuated by the desire to supply what appears to me to be a real deficiency ; and in doing so I have attempted, however inadequately, to co-ordinate or give a connected account of various groups of ascertained facts which hitherto have not been brought into relation. For this purpose I have had occasion to refer to many books and memoirs dealing with subjects that at first sight might have been supposed to differ considerably. Thus, works on zoology and anatomy, obstetrics and gynecology, physiology and agriculture, anthropology and statistics, have been consulted for such observations and records as seemed to have a bearing on the problems of reproduction.

My sources of information are duly acknowledged in the footnotes, but I am glad to take this opportunity of mentioning the following works from which I have obtained special help : “The Evolution of Sex,” by Professors Geddes and Thomson, “ Obstetrics,” by Professor Whitridge Williams, the sections on

A the male and female reproductive organs, by Professor Nagel and Dr. Sellheim, in Professor Nagel’s “ Handbuch der Physiologie des Menchen,” “ Experimental Zoology,” by Professor T. H. Morgan, and the writings of Mr. Walter Heape.

The present volume is addressed primarily to the trained biologist, but it is hoped that it may be of interest also to medical men engaged in gynecological practice, as well as to veterinarians and breeders of animals. As a general rule, I have confined myself to the physiology of generation among the higher forms, and more particularly the Mammalia, but I have not hesitated to discuss the reproductive processes in the Invertebrata in cases where they seemed likely to elucidate the more complex phenomena displayed by the higher animals. The all-important questions of heredity and variation, although intimately connected with the study of reproduction, are not here touched upon, excepting for the merest reference, since these subjects have been dealt with in various recent works, and any attempt to include them would have involved the writing of a far larger book. Similarly, the subject matter of cytology, as treated in such works as Professor Wilson’s volume on the cell, is also for the most part excluded.

It may be objected that, for a book on physiology, too much space is devoted to the morphological side of the subject. This has been done purposely, since it seemed impossible to deal adequately with the physiological significance of the various sexual processes without describing the anatomical changes which these processes involve.

In preparing this work I have been assisted by many friends. I have been fortunate in securing the co-operation of Dr. William Cramer and Dr. James Lochhead, of the University of Edinburgh. Dr. Cramer has contributed the section on the biochemistry of the sexual organs, while Dr. Lochhead has written the chapters on foetal nutrition and the metabolism of pregnancy, a labour of no inconsiderable magnitude in view of the complexity of the subject. I take this opportunity of recording my indebtedness to Mr. Walter Heape, through whose influence T was first led to realise the importance of generative physiology both in its purely scientific and in its practical aspects. I am under no light obligation to Professor

INTRODUCTION 3

Schafer for valuable and ready help at all stages in the preparation of this volume. Professor Schafer has kindly looked through the manuscript of the chapter on “ The Testicle and Ovary as Organs of Internal Secretion,” besides giving helpful advice and criticism on various points connected with publication. Indeed,.it is not too much to say that had it not been for him, the book would scarcely have been written. Dr. H. K. Anderson and Professor Sutherland Simpson have read the manuscript or proofs of the chapter dealing with “The Accessory Male Organs.” Mr. E. 8. Carmichael, of the Royal Infirmary, Edinburgh, has read the section dealing with parturition. Dr. J. H. Ashworth has looked through the chapter on “ Fertilisation ” ; and Dr. F. G. Hopkins has done the same for Dr. Cramer’s biochemical chapter. Dr. Anderson and Dr. Ashworth have also given me the benefit of their special knowledge in other parts of the work. To all these I am under obligations. I wish also to tender my thanks to those authors and publishers who have kindly allowed me to-reproduce illustrations from their respective works, as well as to record my indebtedness to the following, who have been of service by giving me information, important references, or assistance in other ways :—Dr. Nelson Annandale, Superintendent of the Indian Museum, Calcutta, Dr. W. Blair Bell, of Liverpool, Mr. Eagle Clarke, Superintendent of the Scottish National Museum, Professor J. C. Ewart, of the University of Edinburgh, Professor J. P. Hill, of University College, London, Dr. A. C. Haddon, of Christ’s College, Dr. W. A. Jolly, of the University of Edinburgh, Dr. Janet E. Lane-Claypon, of London, Mr. D. G. Lillie, of St. John’s College, Mr. K. J. J. Mackenzie, of Christ’s College, Mr. F. A. Potts, of Trinity Hall, Dr. C. G. Seligmann, of London, and Mr. A. E. Shipley, of Christ’s College. Lastly, I wish to acknowledge the assistance of Mr. C. H. Crawshaw, of Christ’s College, in the correction of the proofs, as well as to express my obligations to Mrs. Hingston Quiggin for the willing labour she has expended in preparing the index and finally revising the text, and to Mr. Richard Muir for the skilful manner in which he has executed those drawings which are new.

Chapter I

«To everything there is a season, and a time to every purpose under the heaven.” —Ecelesiastes iii. 1.

“Tr is well known that almost all animals, except Man, have a stated season for the propagation of their species. Thus the female cat receives the male in September, January, and May. The she-wolf and fox in January; the doe in September and October. The spring and summer are the seasons appointed for the amours of birds, and many species of fishes. The immense tribe of insects have likewise a determinate time for perpetuating their kind; this is the fine part of the year, and particularly in autumn and spring. The last-mentioned class of beings is subject to a variation that is not observed in the others. Unusual warmth or cold does not retard or forward the conjunction of birds or quadrupeds; but a late spring delays the amours of insects, and an early one forwards them. Thus it is observed that, in the same country, the insects on the mountains are later than in the plains.”

The foregoing quotation from Spallanzani’s “Dissertations,” } although not strictly accurate in all its statements, contains a clear recognition of two fundamental facts which indeed have been realised from the earliest times; firstly, that the periods of reproductive activity among the great majority of animals (not to mention plants) occur rhythmically, the rhythm having a close connection with the changes of the seasons ; and secondly, that the reproductive rhythm is liable, to a greater or less extent, to be disturbed or altered by climatic or other environmental influences. And while there may be a basis of truth for the statement that the periodicity of the breeding season

1 Spallanzani, Dissertations relative to the Natural History of Animals and Vegetables. Translated from the Italian, vol. ii., London, 1784, 4 in the higher animals is less liable to modification than is the case with certain of the lower forms of life, there is abundant evidence that among the former no less than among insects the sexual functions are affected by external conditions and food supply.

Darwin remarks that any sort of change in the habits of life of an animal, provided it be great enough, tends in some way to affect the powers of reproduction. “The result depends more on the constitution of the species than on the nature of the change ; for certain whole groups are affected more than others ; but exceptions always occur, for some species in the most fertile groups refuse to breed, and some in the most sterile groups breed freely.” “Sufficient evidence has now been advanced to prove that animals when first confined are eminently liable to suffer in their reproductive systems. We feel at first naturally inclined to attribute the result to loss of health, or at least to loss of vigour; but this view can hardly be admitted when we reflect how healthy, long-lived, and vigorous many animals are under captivity, such as parrots and hawks when used for hunting, chetahs when used for hunting, and elephants. The reproductive organs themselves are not diseased ; and the diseases, from which animals in menageries usually perish, are not those which in any way affect their fertility.” 1

It would seem probable that failure to breed among animals in a strange environment is due not, as has been suggested, to any toxic influence on the organs of generation, but to the same causes as those which restrict breeding in a state of nature to certain particular seasons, and that the sexual instinct can only be called into play in response to definite stimuli, the existence of which depends to a large extent upon appropriate seasonal and climatic changes.”

There are at present no sufficient data for a comparative account of the physiology of breeding among the lower animals ; and in the present chapter, which is preliminary in character, I shall content myself with stating a few general facts about

1 Darwin, Variation of Animals and Plants, Popular Edition, vol. ii., London, 1905.

2 See especially page 20, where Bles’s observations on the breeding habits of Amphibia are referred to.

the breeding season, giving illustrations, taken from various groups of Vertebrates and Invertebrates, of its seasonal recurrence, and the manner in which this varies under altered conditions of life.

PROTOZOA

Among the Protozoa the organisms pass through successive phases of vitality, which are comparable to the different ageperiods of the Metazoa. In such simple forms of life, fission or division into two parts is the usual method of reproduction,’ and the frequency of its occurrence appears to depend more upon the phase which has been reached in the life-cycle, than upon the influences of the environment. Thus, there is a period of extreme vigour of cell-multiplication, corresponding to the youth of a metazodn; secondly, there is a period of maturity, characterised by changes in the chemical and physical properties of the cell, and leading to the formation of conjugating individuals; and finally, in forms which do not conjugate, there is a period of senescence which ends in death. It is interesting to note, however, that the rapidity of fission is affected by the temperature and the food; for example, an individual of the Ciliate Infusiorian, Stylonychia pustulata, if well supplied with food, divides once in twenty-four hours in a temperature of from 5° to 10° C., and once in twelve hours in a temperature of from 10° to 15° C2 Again, Flagellate Infusoria of different kinds have been induced to conjugate by changing the temperature or increasing the density in the surrounding medium.? Furthermore, the life-cycle of Paramecium may be renewed without the occurrence of conjugation, that is to say, fission can be made to continue and senescence can be avoided, by introducing a change in the

* In this process no material is lost, and two simple nucleated organisms result. During the period of maturity referred to in the text, multiplication is often preceded by union (either temporary or complete) of two individuals, and this process is called conjugation (see p. 211, Chap. VI.).

® Sedgwick, Student’s Text-Book of Zoology, vol. i., London, 1898,

5 Calkins points out that the same experiment is performed by mosquitoes and other insects on certain parasitic Protozoa, as when a parasite is withdrawn from the hot environment of the Mammalian blood into the comparatively cold region of the mosquito’s alimentary tract. (“The Protozoan Life-Cycle,” Biol. Bull., vol. xi., 1906.)

composition of the medium surrounding the culture! (See p. 213.)

Moreover, there is evidence that in the case of Colpoda steint at least the occurrence of conjugation is determined entirely by the conditions of the surrounding medium.

C@LENTERATA

With the majority of the Metazoa, as already indicated, there is a more or less definitely restricted season to which the occurrence of the chief reproductive processes is confined.

Thus in the common hydra of Bengal (Hydra orientalis, Annandale), which, like most other Ccelenterates, reproduces by budding as well as by the sexual method,” the former process occurs chiefly during winter, the buds developing into new individuals. Towards the beginning of the hot weather budding becomes less active, and in some individuals ceases altogether, while the same thing happens during periods of temporary warmth in winter. A rise in temperature induces a proportion of the individuals present in an aquarium or pond to develop testes or male reproductive glands; if the rise is considerable it may cause a few of the remaining individuals to produce ova. On the other hand, no individual living in its natural environment has been known to exhibit any sign of sex after the rise in temperature had become steady. The conditions most favourable to the production of ova appear to be a period of comparatively low temperature and abundant nutrition followed by a sudden but not excessive rise of temperature.®

4, Calkins, loc. eit.

2 Asexual reproduction is of very common occurrence among the majority of the lower animals and plants. It may take the form of simple binary fission (in unicellular organisms), of spore formation, or of germination or budding. Sexual reproduction consists essentially of the union of two cells and their subsequent division to give rise to the new individual. In the multicellular organisms (Metazoa and Metaphyta) there are two kinds of conjugating cells, or gametes, which are specialised for the purpose. These are produced by the male and female respectively, and are known as spermatozoa and ova. Thus, sexual reproduction in the Metazoa is a modification of conjugation in the Protozoa. (See Chap. VI.)

3 Annandale, “The Common Hydra of Bengal,’ Memoirs of the Asiatic

Society of Bengal, vol. i., 1906. Cf. Whitney, “The Influence of External Factors in causing the Development of the Sexual Organs in Hydra viridis,”

Some of the marine hydroids show an alternation of generations which does not appear at first sight to be in any way related to change in the environment. In such cases the fertilised ovum develops into a polyp which gives rise to a colony of polyps by a process of sexual reproduction. After the colony has reached a certain size, a new kind of bud is formed, and this becomes a jelly-fish. The latter, after leading an independent existence, produces eggs, and these in turn become fertilised, giving rise to a new generation of polyps. Morgan points out that as the polyp colony goes on increasing in size, its relation to its surroundings must undergo change, and that, very possibly, it is this change which determines the development of jelly-fish in place of polyps. Jf this interpretation is correct the breeding season among marine hydroids is controlled by environmental conditions, just as it is among most other animals.1

Some interesting observations have been recorded by Ashworth and Annandale? about the breeding habits of seaanemones. The species Sagartia troglodytes and Actinia mesembryanthemum, which are very prolific in captivity, have been noticed to breed regularly in the early spring. Actinia commences to produce young in the beginning of February, and Sagartia about a month later. As a rule the young are extruded in the early morning, and one individual may repeat Arch. f. Entwick.-Mechanik, vol. xxiv., 1907. Whitney says that in Hydra viridis an abundance of food following a low temperature causes a suppression of the formation of testes and ova.

1 Morgan, Experimental Zoology, New York, 1907, Morgan shows that the same point is illustrated by certain recent experiments: of Klebs on flowering plants. These at first produce only leaves and branches. When they reach a certain size they produce flowers. Klebs regards the development of the flowers as beiug due to a relation that becomes established between the plant (when it has reached a certain stage of growth) and the environment. He shows also that by altering the environment a shoot may be induced to go on growing vegetatively, when it would ordinarily develop into a flowering branch. The flowering of the plant, therefore, is not merely the culmination of its form, as most botanists regard it. For much valuable and suggestive information on the factors which control breeding in plants G. Klebs’ work should be consulted. ( Willkirliche Entwickel gstind Pflanzen, 1903.)

® Ashworth and Annandale, ‘Observations on some Aged Specimens of

Sagartia troglodytes, and on the Duration of Life in Ccelenterates,”’ Proc, Roy Soc. Edin., vol. xxv., 1904. i rungen bet the process every morning for a number of weeks, when the breeding season comes to an end. In one season, when the aquaria were somewhat neglected, the specimens of Sagartia produced fewer young than usual, and these were not extruded until the beginning of April. Specimens of Actinia living in the same aquaria were more prolific, but their breeding season was also somewhat retarded. In the month of August two anemones of the species Sagartia troglodytes were brought from Thorshavn in the Feroés, and placed in the aquaria. In the following October both of these produced several young; while in April of the next year one of them again gave birth, but only to a single anemone. It seems probable that in this case the change of temperature or environment had induced the anemones to breed at an unusual season; for it is unlikely that October is the normal period for reproduction in the Feroés, as by this time the sea has already begun to run high, and there would be a great risk of the young anemones becoming destroyed, being unable to attach themselves.

Ashworth has pointed out! that in the coral Xenia hicksoni, which lives in the tropics, there is every evidence that spermatozoa are discharged over a very considerable period, if not practically throughout the whole year, whereas in the related form Alcyonium digitatum, of Northern Europe, the period during which the spermatozoa are discharged is limited to about a month in the winter. Ashworth remarks that the difference is probably due to the fact that Xena, living on reefs in the shallow waters of tropical seas, is not subject to great variations in temperature and food-supply, while with Alcyonium, such variations are no doubt considerable. In a similar way Miss Pratt, who has studied the process of odgenesis in Sarcophytum, Holophytum, and Sclerophytum, concludes that the sexually mature condition in these tropical genera extends over a considerably longer period than in the case of corals inhabiting temperate waters.

It may also be noted that, whereas in the Ctenophora of the Mediterranean the breeding season extends throughout the

1 Ashworth, “ Structure of Xenia hicksoni,” Quar. Jour. Micr. Science, vol. xlii. 2 Pratt, “On Some Alcyonidz,” Herdman’s Ceylon Reports, vol. iii.

year, in members of the same class in northern seas it only lasts through the summer.’

NemMERTEA, &C.

The breeding season and its relation to the environment have formed the subject of a careful investigation by Child * in the case of a small Nemertean, Stychostemma asensoriatum, which is found very abundantly in one of the park lagoons of Chicago. The season extends from May to November or December, according to the temperature of the water. Egg-laying can occur freely in the laboratory, the eggs being deposited always during the night, or in darkness, when the animals move about freely. Although breeding in the natural state is restricted to the warmer part of the year, eggs can be obtained in the laboratory at practically any time, by simply regulating the temperature. Thus egg-laying can be induced in the winter at ordinary room-temperature, even though the worms are kept without food. “In animals which contained only a few small odcytes when taken, and which are kept in clean water without food, the growth of the odcytes will continue, and within a week or two eggs may be laid.” “The body of the animal may even decrease somewhat in size during the growth of the odcytes.” It is clear, therefore, that in Stychostemma the limits of the breeding season are determined chiefly by the temperature of the water, and that food is a factor of secondary importance.

Sumilarly, in the case of the parasitic Trematode, Diplozodn paradoxum, which ordinarily produces eggs only in the summer, it has been found that the formation of eggs could be artificially prolonged throughout the winter, if the fishes on whose gills the animal lives are kept in an aquarium at summer heat.?

ANNELIDA

Certain species of Polychet Annelids, known as the Palolo worms, exhibit a quite remarkable regularity in the periodicity of their breeding habits. During their immaturity all the Palolos

> Bourne, “The Ctenophora,” Treatise on Zoology, vol. ii., London, 1900.

® Child, “ The Habits and Natural History of Stychostemma,” American

Naturalist, vol. xxxv., 1901. 3 Semper, Animal Life, London, 1881.

live in burrows at the bottom of the water. With the attainment of sexual maturity, and under certain peculiar conditions, they swarm out for purposes of breeding. In the Atlantic Palolo (Eunice fucata) and the South Pacific Palolo (Hunice viridis) the process invariably takes place twice, upon or near the day of the last quarter of the moon; but with the former species it occurs in June and July, and with the latter in October and November. In the Japanese Palolo (Ceratocephale osawat) the swarming takes place on nights closely following the new and full moons (t.e. when the spring tides occur), in October and November, the worms swimming out regularly four times a year. Hach swarming-period lasts from one to four days. It has been noted further that the swarm is greater after the new moon (when the spring tide is highest) than after the full moon (when the tide is not so high), that each swarming takes place invariably just after the flood in the evening, that it continues for from one to two hours, and is generally larger on warm, cloudy nights than on clear, chilly nights. It would appear also that no individual worm takes part in more than one swarming in the year.1 ARTHROPODA

Innumerable instances of the periodicity of breeding and its relation to seasonal and environmental changes might be adduced from the great group of Arthropods, but the reason for the variations which occur is not always obvious. Thus, in the common crayfish (Astacus fluviatilis), in France the males are said to approach the females in November, December, and January, whereas in England they begin to breed as early as the commencement of October, if not earlier.2 Also, in the Cape species of Peripatus (P. capensis) birth takes place in a fixed season (during April and May), whereas, in the South American species, births are said to occur probably throughout the entire year.*

In the case of the hemipterous insect known as the plantlouse (Aphis), we have evidence that the mode of reproduction

‘ Izuka, ‘‘ Observations on the Japanese Palolo,” Jour. of the College of Setence, University of Tokyo, vol. xvii., 1903.

2 Huxley, The Crayfish, London, 1880. 3 Sedgwick, ‘‘ Peripatus,” Camb. Nat. Hist., vol. xii,, London, 1901.

is dependent upon temperature. In a favourable summer the females of this animal may produce as many as fourteen consecutive generations of young by parthenogenesis, the ova undergoing development without being fertilised by the male. At the beginning of the winter male plant-lice make their appearance and fertilise the eggs, which develop in the succeeding spring. Réaumur, however, by artificially maintaining a constant summer temperature, succeeded in producing more than fifty parthenogenetic generations of plant-lice, all descended from a single mother.?

Morgan, however, describes some observations which seem to indicate that the change is not merely due to temperature. He shows, for example, that the sexual forms of Aphis may appear in the autumn before the onset of the cold weather, and conversely that many individuals may continue to reproduce parthenogenetically, until finally they perish from the cold. Morgan suggests that the alternation in the mode of reproduction may depend upon changes which take place in the food-plant in the autumn, instead of being solely a temperature effect. He shows also that there is evidence for the conclusion that in the genus Chermes, in which the alternation of generations occurs between the fir-tree and the larch, the conditions existing on the larch are those that call forth the sexual forms.?

It has been supposed that the change in the environment is also responsible for determining the sexes in aphids. Miss Stevens, however, has recently shown that what appears to be a change in sex should rather be regarded as a change from the parthenogenetic to the sexual mode of reproduction.’ According to this view the sex of each individual is determined by the character of the gamete or gametes by which it is developed. The supposed influence of food and external conditions upon sex-determination in various kinds of insects, and other animals, is discussed at some length in a future chapter of this work (Chapter XV.).

Semper pointed out long ago that the occurrence of Teproduction (or of the particular mode of reproduction), with

1 Semper, loc. cit. 2 Morgan, loc. cit.

* Stevens, “Studies in the Germ-Cells of Aphids,’

” Carnegi ituti Report, Washington, 1906. he Sarin insects as with other animals, depends, among other things, upon the nature of the diet, upon the chemical conditions of the surrounding medium, upon the moisture of the air, or upon other circumstances which are often unknown. Thus, failure to breed in a new environment is experienced by many Lepidoptera. For example, Death’s-Head hawk moths, which are commonly blown over to this country from the Continent, but do not breed here, deposit their eggs on young potato plants, and these develop into moths which emerge in the autumn. The eggs, however, are quite infertile, so that, as a result, the Death’s-Head has never established a footing in Britain, though stray specimens are often captured! In the case of other insects, such as the mosquito (Anopheles), there is direct evidence that food is an important factor in egg-formation. Thus it was found that mosquitoes fed on bananas refused to breed, but when fed on human blood they invariably laid eggs after two or three days.” It is interesting to note also that in the mosquitoes and other Culicide, the males are generally unable to suck blood, this habit being apparently correlated with the function of oviposition. Dr. Gordon Hewitt informs me that among the Empide, which are carnivorous, the females, during the nuptial flights, are always fed by the males on small insects, and that they seem incapable of discharging their sexual functions unless they are fed in this way.3

In some insects oviposition takes place long after the death of the males. Thus, Lefroy and Howlett state that in the mango weevil (Cryptorhynchus gravis) the males die in August while the females live until the following March to lay eggs.*

MoLiusca

Among the marine Mollusca, in curious contrast to so many forms of life, winter is the usual time for the deposition of the

1 Country Side, October 27, 1906.

2 “Report of Malaria Expedition to Nigeria,” Liverpool, Trop. Med. Memoir, IV. See also Ross (Nature, vol, lxxx., 1909), who says that females of Culex and Stegomya apparently only desire to suck blood after fertilisation.