Paper - The morphology of the external genitalia of the mammala

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Wood-Jones F. The morphology of the external genitalia of the mammala. (1914) Lancet. 1017-1023.

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This historic 1914 paper by Wood-Jones describes the morphology of the external genitalia of the mammala.

There is a second paper referenced for pages 1099-1103.

Wood-Jones. F.. 1914. The morphology of the external genitalia of the mammala. Lancet. 1017-1023; 1099-1103.

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The Morphology of the External Genitalia of the Mammala

Delivered at the Royal College of Surgeons of England on March 4th and 6th, 1914,

By Frederic Wood Jones, D.Sc.Lond.,

Lecturer to, and Head of Anatomical Department at, The London School of Medicine for Women.

Lecture I.

Lecture I.

Delivered on March 4th.

Mr. President, Ladies, And Gentlemen. - One of the natural outcomes of the efforts of naturalists to organise the study of the lower forms of life has been the desire, manifested in many different ways, to classify and arrange the vast wealth of material which is available for study. The orderly arrangement of the living world into properly labelled classes has been the aim of a very long line of students of nature. There is an attraction about this marshalling into ordered array which has engaged the attention of even the greatest of naturalists; has limited the activities of lesser -ones; and has cramped, confined, and stultified the whole life work of some of the least among them.

Systems have come and gone; first one set of attributes and then another has been claimed as furnishing the true guide to the position of a living thing in the vast assemblage of the forms of life. It is not proposed to follow the rise and fall of the many methods of classification, for it is only with the comparatively modern that the present investigation has any concern. It is assumed that the system of classification into the fabric of which an attempt has been made to weave one definite thread is, as Huxley expressed it some 40 years ago, “based upon purely structural considerations, and may be designated a morphological classification.” Following this guide we may limit ourselves to a «consideration of the arrangement of living things by a study of their structure.

Structure and Function

Now the “ structure ” of an animal or plant consists in the ordered disposition of an assemblage of -organs to constitute one composite body, and it is therefore obvious that any classification by structure is in reality a classification by organs or the assemblage of organs. Of the work of John Hunter, Huxley said, “ the classification which he adopted is a classification by organs,” and that is true ; but it must not be lost sight of that the summation of ” organs ” constitutes “structure.” This fact, I think, has been somewhat overlooked, indeed, Huxley himself seems to have been inclined to ignore it when he said of John Hunter’s system, “it is admirably adapted to the needs of the comparative physiologist.” Looked at in the way I am endeavouring to present the problem, Hunter’s "system is equally adapted to the needs of the comparative anatomist. Huxley also observed truly that the unrivalled collection of John Hunter “ was intended to illustrate the modifications which the great physiological apparatuses undergo in the animal series"; and here, in the midst of this collection. it is unnecessary to remind anyone that Hunter — more than any other anatomist- demonstrated that the whole form and disposition of organs is dependent on the function of the organs. We have therefore come back — as every excursion in search of truth in living economy will inevitably bring us back - to the starting-point that “ structure depends upon “function.” Unfortunately, such reasoning also leads to the apparently hopeless conclusion that since classification is effected by consideration of structure, and structure is dependent on function, classification itself depends ultimately on function. It is, however, only when dealing with the simplest forms of life that the very paucity of organs in which to note differences and likenesses allows us to see that the systematist is in reality separating and grouping his material by reference to function.

It is the increasing complexity of the assemblage of organs — and that alone — that saves the museum specialist from having to make the humiliating I confession that he is endeavouring, under the worst possible conditions, to arrange his specimens according to the functions which they possessed during life. Hand in hand with increasing complexity there is manifested a wonderful conservatism in the living type which admits of funcI tional alteration of structure in one organ or system of organs, while some other system retains its lancestral and primitive condition. It therefore [comes about that, functional considerations producing alterations in any one system, it is by a comprehensive study of the whole of the organs of an animal that classification is achieved. It must be apparent, however, that such a method is only to be regarded as thoroughly reliable on the assumption that no functional needs can affect a complex living thing so profoundly as to alter every part of its body in harmony. It must also be noted that it is only by the complete knowledge of the entire anatomy of any form that real certainty of its position in any scheme can be secured.

How completely function may modify such a complex thing as the structure of a mammal we do not know; but there is evidence that the completeness with which it may do so may be a pitfall to the systematist. A well-marked and peculiar specialisation - such, for instance, as the dependence upon ants as a source of food - has produced profound alterations of structure in a large number of animals. It was only precise anatomy as distinct from mere superficial examination that separated Echidna from the Myrmecophagidee with which it was originally placed; and it is still open to question if the Icommon habit of eating ants rather than true zoological affinity is not the real bond of some of ‘the divergent forms grouped together as the Edentata. Again, a completely aquatic life has produced far-reaching modifications of the whole I body of many creatures. And though no one with any anatomical knowledge would be in danger of lclassifying the dugong and manatee with the whales or with the seals, still it may be doubted if the modifying effects of such a life have been properly realised in separating the Sirenia so far from the elephants and tapirs. In these instances there has been an alteration in the structure of many systems in response to the same functional need, but there are still enough organs left retaining their ancestral condition to put the true afiinities of the animals beyond doubt. But it is, perhaps, not beyond the range of possibility to imagine that the modifications could become more complete, and that very little indeed might retain its primitive characters.

Trends of Thought in Classification

The fact that functional modifications of any one set of organs may render them unsuited to the purpose of universal classification has produced two definite trends in the study of living forms. In the first place, it has resulted in the specialist in one restricted group selecting the characters of one set of organs for his criteria of classification, while the worker in another, and perhaps nearly allied, group arranges his material by reference to an entirely different set of organs. This inconstancy of standard is apt to strike an amateur and unspecialised student of nature rather than a highly trained specialist. In the second place it has resulted in compelling the man who takes a more comprehensive view of living things to embark on a search for the underlying “ type” which is modified, here in one direction, and there in another. It is unnecessary to follow the evolution of the theory of an essential plan which, old as Plato, received a fresh stimulus from the work of Goethe, and culminated in one direction with the researches of Richard Owen and others which produced the conception of the “ archetype.”

These two trends of thought must be briefly noted. Concerning the second, it is frankly outside the province of this paper, although it provides a most interesting study. Plan—or ancestral type—— of course there is; it was almost the perfection of the development of this idea that brought about its relegation to the background of scientific thought. The “archetype” grew so complex that it became more a creed than a part of knowledge. The criticism of Herbert Spencer decreased its vitality; the advent of the ideas of Darwin caused it to be neglected, and added to its decrepitude, and the study of embryology has reduced it to-day to the position of a dead ideal of a past generation. And yet the death is no more than apparent, for its own offspring has demonstrated its vitality. As Dwight has said, “embryology, which 50 years ago was in its infancy, confirms distinctly the view of the archetype.” Even if we abandon the use of the many terms and phrases that grew up around this central idea, we must still retain the fundamental principle involved. We have shifted our ground rather from the notion of an adult “unity in variety” to the conception of an embryological recapitulation.

With the other tendency of efforts at classification we have, however, more direct concern.

Criteria of Classification Of Vertebrates

For the moment we will pass by the organs and systems most used in the comparison of the lower forms of life and turn at once to the vertebrates, and especially the mammals with which this paper deals. The most superficial examination of the methods used to determine aflnities among the representatives of higher orders reveals the extent to which reliance is placed upon features of the skeleton, the teeth, and the integument. This is, of course, owing to the fact that these are the parts most readily capable of preservation, and therefore most commonly available for study by the museum specialists who are responsible for most classifications. Again, since place has to be found in any scheme for the material contributed by the pale ontologist, it is essential that bones and teeth should be accepted as criteria of rank. The question is, Are these undoubtedly convenient, and often sole available criteria, ideal ones upon which to base amnities ?

The Skeleton

If we regard the skeleton, as some anatomists have apparently regarded it, merely as a sort of scaffolding upon which the body is built, then we may feel comparatively safe in presuming that similarities in the design of this scaffolding will point to relationships between the bodies of which the scaffoldings were the frame work. But if we regard the osseous system as a highly plastic set of organs, which shows a wonderful ability to adapt itself, by beautiful adjustments, to every demand of stress or strain resulting from each functional poise and purpose of the limbs and trunk, then we shall have doubts as to the validity of these assumptions. Every modification of gait, every alteration of muscular adaptation produces its change.

habits: swimming, running, climbing-, and burrowing all bring about modifications regardless of


zoological afinities. Even the skull is profoundly altered by such simple functional factors as the method by which food is conveyed to the mouth. Numerous instances of skeletal adaptability will occur to every one; but one, which bridges over such wide spaces that real confusion would be impossible, may be given. The tarsometatarsal of typical birds is an elongated bone into which the elements of tarsus and metatarsus have become fused. In the penguins (Spheniscidee), which walk or waddle in mammalian fashion, the elements are partially separated and the composite bone is short as in the mammals; in the jerboas (Dipodinee), which hop like birds, the avian proportions, and even the fusion of the elements, are present. This is an extreme instance that is sogross that it could never deceive ; but it shows how bones adapted towards the same functional end could possibly be taken as indicating zoological affinities between forms which, though widely separated, are members of the same order.

The Teeth

It is the same with teeth; for these organs subserve the all-important function of dealing with the food of the animal, and, in addition, may be developed for killing prey, for catching prey that needs no special killing, for cutting herbage, for fighting rivals, as tools employed in life processes unconnected with feeding, and even as organs of locomotion. Now, of course, any organ connected with the feeding habits of an animal»-in fact, any part of the alimentary system—~—must be expected to vary in response to the nature of the food taken, and the method employed in taking it. A wide series of facts fulfils this expectation. It is recognised and admitted at the outset that far more than mere functional adaptations are considered in any classification by teeth; still, it is impossible to escape the conviction that too great a reliance upon the characters of these organs may very easily mislead.

The Integumentary System

With the integumentary system it is unnecessary to deal at length; it is the happy hunting ground of the systematist. Colour, pattern, a few hairs more or less here or there, a stripe or a spot or so, have often been enough; as we might make a red head or a bald one, a grizzled beard, or a freckled face the criteria for species-making among mankind, so has the systematist used the integumentary system. There is something far more subtle in the outer covering of an animal than such treatment can take into account. As a huge sense organ, as a reflecting surface of the central nervous system, and as a medium for the display of everything for which the life processes of the animal demand a canvas, the skin of every living creature must be considered. Among the Edentata alone we have animals with a normal hairy covering (Myrmecophagidae), animals with coats of specialised hair (Bradypodidse), with skins the condition of which has prompted the familiar name of “pigs” (Orycteropodidse), with armour plates (Dasypodidee), with protective shells (Grlyptodontidse), and with imbricated horny scales (Manidee). In the integumentary system, again, we even have the strange fact that so conspicuous a part as the tail may become entirely lost as a normal process in functional life. The aberrant porcupine (Trichys), found in Borneo, starts life with a tail, as T. guenthe/rel, but the female, at any rate, loses it, and ends as the tailless T. Zelpuxra.

Systems of Varying Degrees of Constancy

Passing at once from these systems which, owing to the varying ways in which the functions that they subserve may be carried out, are plastic, and ever ready to be shaped in the varying mould of habit, to those organs which possess one function carried out on constant lines, we note immediately an enormous difference.

The teeth have to deal with food which may be of almost any character, obtained in almost any way; but the essential organ of sight or of hearing subserves one definite function effected in one constant way throughout the whole series of animals. Animals see or hear in only one way; they feed, progress, breathe, and become protected by colour, armour, or pattern in a very great number of different ways.

The constancy of the structure of the internal ear, even in widely divergent forms, has been noted by many workers, among them especially by Dr. Albert Gray; the same authority has noted the greater variability of the middle ear, while the external ear is the delight of the systematist, so many are its modifications.

The whole optic apparatus, including its accessory parts, is profoundly modified by function; but the essential organ of vision is so remarkably constant in its structure that though one might make a shrewd surmise that the eye belonged to an animal of diurnal, crepuscular, nocturnal, or subterranean habit, it would tax the powers of the systematic zoologist to assign an excised eyeball to a member of any of the main divisions of the mammalia.

It would seem, from such a point of view as I am attempting to take, that in the animal body there are systems with highly variable functions, and consequently with such highly variable characters as to form unreliable criteria for any classification other than a frankly functional one. Again, it seems there are systems of such constancy of function, and consequently of such uniformity of structure, as to furnish no clues to the finer gradations of affinity such as are required for a satisfactory scheme of classification. It is not likely, and it is not reasonable to expect, that there is any one system in the animal economy which would strike the mean between these two extremes and form in itself an absolute and safe standard by which to assign an animal to its proper place in the scheme of living things. Yet there are undoubtedly some systems which have special claims in this direction.

The brain has great title to be regarded as an organ which notes the rank of its possessor. Since it is true, in a great measure, that all evolution, the whole scale of animal types, consists in an increasing perfection of the brain, it may be said that the state of the brain will indicate at once where in the scale its possessor should be placed. The brain may be said to sum up in its complex structure all the functions and attributes of an animal; and increasing completeness of the knowledge of the brain would mean increasing perfection of classification. The work of Elliot Smith especially has made this clear. If we had to depend upon one organ only to provide all the materials for a scheme of universal classification no better choice could be made than the brain.

The Reproductive System

Another system which has long seemed to me to deserve special attention in this connexion is the reproductive system; its functions are so simple, its needs for change so infrequent, and yet the changes that have been produced in it are obviously so profound. The reproductive system has not by any means been passed over as a guide in classification. The botanist places reliance on it; to the zoologist who deals with parasites it may furnish the most valuable guide, since even if most of the other systems of the body become reduced to mere rudiments, the reproductive system must persist in full functional development. Even in the mammalia the characters of the uterus and of the placenta are commonly used to make fundamental distinctions of the greatest importance. “ So far back as the year 1816 M. de Blainville pointed out that the mammalia might be divided into three primary groups, according to the character of their reproductive organs, especially the reproductive organs of the female.” It is from this work of M. de Blainville that we have inherited the divisions of ornithodelphia, didelphia, and monodelphia.

Classification by placental characters does not so directly concern us here, but Huxley has recorded his opinion of the taxonomic value of this structure in the most emphatic terms: “ It appears to me that the features of the placenta afford by far the best characters which have yet been proposed for classifying the monodelphous mammalia.” The study of these portions of the reproductive system has therefore already yielded important results for the systematist, but I do not think that the taxonomic value of the system is exhausted by the study of these two characters. In most instances it is practically certain that a complete study of the foetal and adult genitalia of any debated candidate for classification would clear up all doubts as to its real systematic position.

A detailed study of the development and the adult condition of the genitalia of the whole mammalian series is obviously a stupendous task and one that is limited at every turn by the lack of suitable material. It has therefore been my aim to examine a series of types of which suflcient material could be got together, and to attempt to determine any principle which underlay the modifications that were met with. Put quite briefly, it has been forced upon me that two very different tendencies are seen in the modifications of this system: (1) the marked conservatism of the type upon which the genital system is formed; and (2) the complete transformations of which it is capable in functional emergencies. These tendencies, which at first sight seem to be opposed, need some explanation; and it is necessary to follow, briefly, the modifications of the genital system up to the point at which we commence its detailed study in the mammalia.

We may take as a simple type of vertebrate genitalia, a system which consists, in both sexes, of bilateral genital glands, which may or may not shed their products into the ocelom, but which ultimately pass these products to the cloaca, and so to the exterior, by ducts which are primarily bilateral. Such a system is found, with only minor modifications, throughout the fishes and amphibians; and, functionally, we may say that such a system is found where the ovum is extruded as a more or less gelatinous mass either into water or into some damp place. It is the simplicity of the female genital products, extruded under such circumstances, that has served to retain the simplicity of the genital system. As long as a soft gelatinous ovum is extruded into water so long may the spermatozoon penetrate its envelope and effect fertilisation after extrusion. External fertilisation, therefore, merely requires a system of genital glands and excretory genital ducts by which the genital products may escape to the exterior, and meeting in the surrounding medium insure fertilisation. But such a genital system will only suffice for aquatic animals, or animals which are aquatic or semi-aquatic during their breeding season; for an egg, so delicate that a spermatozoon may penetrate it, is far too perishable an object to be extruded in ordinary circumstances into the Open air.

When an animal becomes completely terrestrial in habit it is essential that the extruded egg cell should have some protective covering to insure it against destruction by the Ordinary agencies that come into play in terrestrial life. It is the addition of this protective covering that produces such great primary changes in the genital system; for the protective covering prohibits the entry of the spermatozoon after the extrusion of the ovum, and internal fertilisation becomes a necessity. This is the first great modification brought about in the genital system-—a modification due to the alteration of life habit.

The vertebrates which first became, in part, independent of their aquatic environment still retained their ancestral habit of laying unprotected eggs, and this being so it was necessary for them to return to the water during the period when they were laying their eggs. It is the breeding season, and that alone, that kept, and keeps, the amphibia in touch with their ancestral environment. Some few have solved the problem by retaining the ovum within the genital passages until some degree of development has taken place (Sa,ZcLmamd7'a extra, Oaecilia compressicaiida, &c.). Such forms must effect internal fertilisation in some way; and the modifications that have taken place especially in Caecilia among the Grymnophiona are of peculiar interest and will need future reference.

Other aquatic animals that have attempted to colonise the land have encountered the same disabilities, and here we may note, as we shall repeatedly have occasion to do, parallel developments in the invertebrates. Among the Isopoda, Bi/rgios Zatro has become apparently a purely terrestrial form, and yet though its whole body is admirably adapted to its new life, its ancestral habit is retained; and once a year it has to return to deposit its eggs in the element from which it has so successfully emancipated itself in all other respects. A whole series of crabs showing intermediate stages of terrestrial adapta tions exists, and in other invertebrate orders instances could readily be multiplied.

Of the vertebrate stock it is the reptiles as a class that first become so completely terrestrial as to be able to deposit eggs on land. Even those members of the class that lead aquatic lives return to the land to lay their eggs. The factor which permits this complete reversal of habit is the addition of the protective covering-the membranous or calcareous egg-she1l»—to the ovum to enable it to withstand the desiccating influences of subaerial incubation. But since this protective membrane is laid down in the female genital passages it is essential that the spermatozoon should reach the ovum when still within the passages and before the protective membrane is ldeposited. With this functional change great structural alterations must be brought about, for I the male must evolve some means for injecting the spermatozoa into the female genital passages, and the female passages must be modified for the I reception of this.

Changes in Structure to Secure Internal Fertilisation

We may see the initiation of these changes in non-terrestrial forms, for the elasmobranchs, though thoroughly aquatic, have found it necessary to lay protected eggs (the familiar “ mermaid’s Ipurses”) or even to retain the ovum and embryo during certain phases of development within the [maternal genital passages. In them fertilisation must of necessity be internal. In response to this lneed the male fish has developed the “claspers” (though the name is by no means a good one) for insertion into the female cloaca. By these claspers Ior seminal guides the spermatozoa are conducted from the male cloaca into the female cloaca and I genital ducts, and internal fertilisation is effected.

This seminal guide becomes the functional type of l the male copulatory organ of the higher terrestrial vertebrates. I We have a parallel evolution among the inverte— brates, many of which lay protected eggs and have to secure internal fertilisation. Among the Insecta, the development of copulatory organs has reached a high state of perfection; and it is to be noted Ithat in many orders these organs have provided the systematist with very satisfactory guides for classification. In the invertebrata again the proIduction of living o""spring has been adopted in many forms. | Although analogous structures, the claspers of the elasmobranchs are morphologically quite distinct from the copulatory organs of the mammals. The first hint of the mammalian condition is afforded in the Grymnophiona and in Salamandra among the Amphibia. This fact has been noted by Gegenbaur, l who says: “ U11 indice d’un organe copulateur se remarque chez les amphibiens (Salamandrines) sous la forme d’une papille faisant saille dans la oloaque.” Gymnophiona has long been recognised as an amphibian which everts a portion of its cloacal wall as a copulatory organ which is some 5 centimetres long.

In the most generalised of the Reptilia (Sphenodon) no definite copulatory organ is developed, and doubtless some everted portion of the cloacal wall suffices as an intromittent organ. But in all other reptiles a specialised copulatory organ is present.

Reptilian Types of Organs

It is of the utmost importance to note that two distinct types of copulatory organs are developed in the reptiles. The first type concerns us most directly, since it is so obviously a stage that is in advance of that shown in the Grymnophiona or in Sphenodon, and yet is so easily interpreted as being a stage that is but little behind that seen in the monotremes, or in the more primitive insectivores among the mammals. This is the type of copulatory organ developed in the chelonians and crocodiles. (See Fig. 1.) The second type—-that belonging to the snakes and lizards-We may dismiss from this inquiry with a brief statement of its general condition. Copulatory organs of this type consist of paired eversible sacs, each furrovved by a seminal groove, Which are devoid of cavernous tissue and which are not developed Within the cloaca. They are extremely reminiscent in some Ways of the claspers of the elasmobranchs and are dificult or impossible to homologise with the copulatory organs of the mammals.

The genitalia of the chelonians and crocodiles, however, need careful study, since they seem so patently to carry on the process which, initiated in the gymnophiona, culminates in the mammalian condition. (See Figs. 1, 2, and 3.)

The marine chelonians possess an intromittent

Penis of a species of Testudo Within the cloaca and exposed by

The condition present in these reptilian types is singularly like that seen in some of the least specialised birds; for the Ratitae possess copulatory organs obviously akin to those described, and other avian orders show rudiments or modifications of this condition. The perfection of this reptilian copulatory organ is seen in some of the larger tortoises, such as Tcstudo clephantina. (Fig. 1.) In these animals the organ is extremely large, and indeed in all the tortoises the mechanical difficulties imposed by the rigid carapace appear to be the functional cause of the great development of the penis. The cloacal orifice is a transverse slit situated at the root of the tail, and in the ordinary quiescent condition the copulatory organ is entirely hidden Within the cloaca, but even in these circumstances a very slight eversion of the cloacal margins reveals the free extremity of the penis in the male. In the female the Whole structure is considerably reduced.

Comparison of Mammalian with Reptilian Organs

In the copulatory organs of these animals all the

FIG, 2, FIG. 3.


The penis of Testudo clephanteina freed from its connexions with the cloacal wall. In the male of this species the “seminal guides” pass within the anal margin. P, Fold over terminal portion (prepuce). G, Terminal portion (glans). U, Seminal groove (penile urethra). F, Seminal guides (inner genital folds). 3., Anus. (From a specimen in the Museum of the Royal College of Surgeons of England.)

of the dorsal wall of the cloaca. The rectum is opened and the “seminal guides.” are seen passing into its lumen. C, Cloacal orifice. U, Seminal groove. P, Penis. F, Seminal guides. S, Uro—genital sinus. R, Rectum. (From a specimen in the Museum of the Royal College of

Penis of a species of Emys. The cloaca has been split into an anterior and a. posterior portion by incisions along its lateral margins, and the anterior wall (penis) turned upwards.

organ that is far more highly developed than anything seen in the amphibians. Specialisation of the ventral wall of the cloaca has resulted in the production of an erectile thickening which is marked by a median groove—-—the seminal groove-———-which runs from the cloacal orifice of the urogenital sinus almost to the distal end of the erectile body. In some forms this ventral erectile mass becomes raised from the cloacal Wall, and altogether free of it at its most caudal limit, and a projecting penis, grooved on its dorsal aspect, is produced. In many tortoises this intracloacal penis becomes further developed. The distal portion Which is free of the cloacal Wall becomes longer, and at the same time specialised, and prominent lips are developed along the sides of the seminal groove.

Surgeons of England.)

S, Orifice of uro—genital sinus. R, Orifice of rectum.

essential parts of the mammalian external genitalia may be recognised. The penis stretches from the urogenital sinus orifice to its free tip just Within the cloacal margin, and the proximal portion of its erectile tissue obtains a skeletal fixation point on the pelvic girdle Within the carapace. It is obviously strictly comparable with the whole of the erectile mass of the mammalian penis. Its dorsal surface «is marked with a groove—the seminal groove——lvvhich is continuous with the urogenital sinus orifice and runs towards the free extremity of the penis, but comes to an end some little distance short of that extremity. Along the margins of this groove bilateral free folds are developed; these are sometimes termed the plicse recto-urethrales, but is seminal guides is the best name that we can give them at this stage. These folds terminate towards the tip of the penis at the point at which the groove ceases; towards the root of the penis they become more prominent and, skirting the sides of the uro—genital sinus, they disappear within the margins of the anus or meet slightly in front of the anus. The free tip of the penis shows several interesting features. The seminal groove and its ridges end at a spongy swollen mass of tissue which is distinctly marked off from the rest of the body of the penis by a deep groove which surrounds its lateral and distal aspects. The free tip of the penis distal to the surrounding groove is a narrow margin of tissue which may be said to constitute a hood encircling the spongy erectile mass in which the seminal groove ends. This terminal hood I imagine may be taken as the ‘chelonian representative of the human prepuce, and in harmony with this the distal spongy mass R appears to be the equivalent of the 3 human glans. The - seminal groove becomes the penile aspect of ~\ C the humanurethra and the fused /1,. seminal guides. constitute its floor. In the female these structures are reduced as a whole; the seminal guides are not so well developed, and it to be noted that, as a rule, they do not extend further back than the orifice of the urogenital sinus. The copulatory organ is smaller in the female and the seminal groove is not so deep as in the male. Of the functional significance of the parts there can be no doubt. The erectile copulatory organ of the male is protruded and everted from the cloaca, and in this action the seminal guides tend to close over the deepened seminal groove and to convert what is, in the unerected condition, a mere depression, into a complete tunnel. As the male copulatory organ is passed into the female cloaca it comes into contact with the dorsal aspect of the female copulatory organ, and it is highly probable that in these creatures the seminal channel is completed by the apposition of the seminal groove and guides of the male with those of the female.

FIG. 4. The cloaca of a chelonian opened to show the cloacal roof and the intracloacal genital tubercle in Sim (semi—diagramrnatic) C M, Cloacal margin. G T, Genital tubercle. S, Uro-genital sinus. R, Rectum.

FIG. 5.

The chelonian cloaca seen from the left side in section. The penis is in the quiescent intracloacal condition. The products of the urogenital sinus in this state are shed into the cloaca. R, Rectum. C, Cloaca. S, Uro-genital sinus. GT, Genital tubercle.

Fig 6

In the ordinary quiescent condition of the parts the products from the male uro-genital sinus merely pass out of Chelonian cloaca seen from the left side in section. The penis in the functional extroverted condition. The seminal guides now meet the orifice of that and convey the products from the uro-genital sinus to the end of the penis cloaca, and so to the exterior. (Fig. 5.) In this special act 1t is necessary that they should be conveyed into the corresponding orifice of the female, and this is effected by the closure of the male seminal guides, possibly assisted by those of the female. (Fig. 6.) Now, between this stage and the condition seen in the Monotremes there is no very wide difference; nor for that matter is there any very sudden transition to the type of cloacal genitalia of some of the Insectivora speaking broadly, the advance made consists of the closing over and meeting of the seminal guides in the male, so that the seminal groove is closed in whole (Insectivora) or in part (Echidna); but the cloaca still exists, and in the quiescent state the external genitalia are still retained within the cloaca.

Fig. 7. A reconstructed section of the cloaca of Crocidura bottigi (shrew from Abyssinia) showing the intracloacal penis. P, Penis. C, Cloaca. R, Rectum.

In echidna the intracloacal genital tubercle of the male presents a completed penile channel in distal portion, but at its proximal end the seminal guides have not completely united over the seminal groove and a state of normal hypospadias exists at the root of the penis, this deficiency being obliterated with erection of the organ. In the cloacal Insectivora, such as Blamlncz. and C'r0c2Idu/rczl, the closure of the seminaxl canal is completed in the male, and the hypospadic condition of Echidna is lost. (Fig. 7.)

We have therefore in some of the mammalian orders a retention of a cloacal condition, and in these an advance has been made on the chelonian condition mainly by the closing in of the seminal groove. In the cloacal Insectivora the penis has reached a high degree of development and is usually remarkable for its great length, so much so that it is retained, more or less coiled, in a diverticulum of the ventral part of the cloaca. It is very important to recognise that the cloacal condition (Zoos exist among the Insectivora, and I have come to think that the appreciation of the differences to be detected in the methods of formation of the external genitalia within the limits of this group forms a basis for a better understanding of the inter-relations of the other mammalian groups. To this point it will be necessary to revert later on. It is equally important to recognise that the chelonian condition is not only practically identical with the lowest mammalian type of external genitalia, but it is also represented as an early stage in the embryonic development of the external genitalia of all mammals. intracloacal genital tubercle, bearing a seminal groove marked laterally by seminal guides, present, the adult external genitalia of all mammalian orders are derived; but the method of derivation and the final condition produced present very striking differences, first, within the limits of the group Insectivora, and second, within the whole compass of the mammalia. It is necessary to describe the stages of the development from this simple condition as they are displayed in various mammalian embryos.

I have come to believe that there are two main types of development and two main types of resulting external genitalia; that all mammals may be placed in the one group or the other ; and that an examination of the adult genitalia will generally, and an examination of the embryonic stages will always, reveal definitely into which group a particular species falls.

Cite this page: Hill, M.A. (2018, December 11) Embryology Paper - The morphology of the external genitalia of the mammala. Retrieved from

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