Human Embryology and Morphology 24

From Embryology

Keith, A. Human Embryology And Morphology (1921) Longmans, Green & Co.:New York.

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures


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Chapter XXIV. Urogenital System (Continued)

Evolution of the Penis

The transformation of the mesonephros to form an adjunct of the genital system of the male is of ancient origin, but those remarkable changes which are seen to occur in the perineum of the human embryo represent a much later evolutionary movement. Even in the lowest mammals — monotremes and marsupials — the rectum and urogenital ducts end in a common terminal passage — the cloaca (Fig. 403, B). In the human embryo, until the 7th week of development, this is also the case ; but about the beginning of this week, when the embryo is 12 mm. long, changes occur which separate the rectal and urinary passages. These changes have been occasioned by the evolution of an external or extra-cloacal penis. In Fig. 403 stages in the evolution of the penis are represented. In the tortoise the penis lies on the pubic or ventral wall of the cloaca ; during copulation the cloaca is partially everted and the open groove of the penis is converted into a canal by the application of the dorsal or opposite waU of the cloaca. In Echidna — a primitive mammal — ^the penis is still intra-cloacal ; its groove is converted into a canal, except posteriorly, where there is still a communication between the urogenital and cloacal passages — which represents the primitive urogenital orifice, for the penile canal is a new passage (Fig. 403, B, 4). In marsupials (Fig. 403, C) the penis is still partially intra-cloacal, but the primitive urogenital orifice is closed, and the urogenital passage is now separated from that which serves for the faeces. In man the penis, as in all primates, has been permanently extruded and is now completely extra-cloacal, and a perineal body separates the rectal orifice from the urogenital passage. The metamorphosis of the cloaca is thus a result of the evolution of the penis. The external penis with a complete penile urethra appears with the evolution of a vagina, uterus and the intrauterine nourishment of the young. The cloacal passage is seen in oviparous mammals ; in viviparous mammals the penis is evolved as an intromittent organ, and the urogenital passage is separated from that of the rectum.

Twofold Origin of the Cloaca

The primitive cloaca, as represented in Fig. 403, A, is of double origin, the deeper part in which the rectum and urogenital sinus end is derived from the hind-gut and is thus of entodermal origin. The more superficial part, enclosed by the cloacal lips, arises as a perineal depression and is thus of ectodermal origin. Students of embryology, however, when they speak of cloaca, have in mind only the part derived from the hind-gut — the entodermal cloaca. The development of the perineal region cannot be understood unless it is remembered that, both ectodermal and endodermal elements play a part in fashioning the anal and vulval orifices'of the human embryo.



1 See articles by Prof. Wood Jones, Journ. Anat. 1914, vol. 48, p, 73 ; 1915, vol. 49, p. 393 ; 1916, vol. 50, pp. 1, 189.

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Fig. 403. Stages in the Evolution of the Penis. A, stage seen in tortoise ; B, stage seen in Echidna ; C, stage seen in marsupial (kangaroo) ; D, stage seen in man. 1, bladder ; 2, Wolffian duct (vas) ; 3, rectum ; 4, urogenital sinus ; 5, anus ; 6, phallic groove and canal ; 7, glans ; 8, cloaca ; 9, cloacal orifice ; 10, floor of phallic canal ; 11, Cowper's glands ; * position of primitive orifice of urogenital sinus.


The Cloaca of the Embryo

Having in the previous chapter traced the origin and fate of the genital ducts, it is now necessary to follow the changes which are undergone by the cloaca — the common vent for the rectum and genital passages. We have already seen that the cloaca appears early in the 4th week (Fig. 274) ; its precocious origin being undoubtedly due to the fact that it gives origin to the allantois, by means of which the chorionic circulation is established. Thus in the 4th week (Fig. 407, A) the cloaca forms a relatively large cavity, into which open the rectum and allantois, while the Wolffian duct is also establishing a communication with its more ventral part. At this time the outline of the cloaca, as seen on making a median section of the embryo, is triangular in outline ; its dorsal wall follows the curve of the notochord to the point of the tail ; a large part of its ventral wall is formed by the cloacal membrane — which is composed of only the two primitive layers — the entoderm which lines the cloaca, and the ectoderm which covers the embryo. It will be remembered (see p. 35) that the hinder end of the embryonic body is produced on each side of the primitive streak. The cloacal membrane occupies the site of a part of the primitive streak, thrust into a ventral position by the outgrowth of the tail (Fig. 404). The hinder apex of the cloaca extends beneath the tail and behind the cloacal membrane and forms that transitory structure known as the tail gut. In the 4th week the cloaca has no perineal opening ; that opening is first established near the end of the 2nd month by an absorption of the cloacal membrane.



^ A. G. PoUman, Amer. Joum, Anat, 1911, vol, 12, p. 1 (Dev. of Cloaca).


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Fig. 404. The Formation of the Cloaca from the Hind-gut during the 4th week. (Wood Jones.) A, section of the posterior end of a liuman embryo early in the 4th week ; B, later in the 4th week when the hind fold is more produced and the cloaca assuming its triangular form.


Evolution of Cloacal Structures

To understand the significance of the changes undergone by the cloaca in the human embryo, one must have first a clear conception of the various evolutionary stages known to the comparative anatomist. We have already seen that some of these changes are related to the differentiation of an external penis ; it is now necessary to see how the cloaca becomes modified to assume its mammalian and human form. The essential stages are represented in Fig. 405 ; in the frog {A) the cloaca receives the bladder, rectum and Wolffian duct, the duct opening distal to the rectum, being thus nearer the cloacal vent. In the tortoise {B) the rectum has passed distal to the Wolffian duct, which now opens with the bladder into a common part of the cloaca — the urogenital sinus (Fig. 405, B, TJG). In the lowest mammals — monotremes (C) — the urogenital sinus has become elongated and assumed the form of a urethra ; the ureter is now severed from the Wolffian duct, but still opens on the floor of the urogenital sinus ; the urine thus has to pass across the urogenital sinus to reach the bladder. In marsupials [D) a further stage is reached ; the cloacal anus of the rectum has migrated backwards on the posterior wall of the cloaca until it almost reaches the perineum. This posterior migration of the rectal opening (anus) is already seen in


Echidna (C), where the urogenital sinus — which represents the proximal part of the cloaca — has assumed a considerable length. Thus in the evolution of mammals we see that the rectum migrates backwards until its vent or anus almost reaches the surface of the perineum, leaving the greater part of the cloaca as the urogenital sinus.

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Fig. 405. Diagrams to show the manner in which the Cloaca is modified and the Termination of the Rectum transferred from the Cloaca to the Perineum in Higher Vertebrates. A, the amphibian form; 1, bladder; 2, Wolffian duct (ureter and vas) ; 3, cloaca ; 4, rectum; 5, intra-cloacal anus ; 6, cloacal orifice ; M.D., MiilJerian duct ; B, form found in the tortoise ; C, form in monotremes; D, form found in female marsupial ; X, floor of urethra.


Ectodermal Cloaca

The forms of cloaca depicted in Fig. 405 are not entirely derived from the gut or entodermal cloaca, which is seen in the human embryo. The terminal or superficial part is derived from a cloacal depression or pit formed on the perineum, and lined by epithelium derived from the ectoderm. The glans of the penis and also of the clitoris are formed in the ectodermal part : the rest of the penis and clitoris is developed in the entodermal part (Fig. 403). We have already seen how the urethral groove of the cloacal penis becomes closed ofi as a separate channel by the union of two vestibular folds — seminal guides Prof. Wood Jones has named them — ^the penile urethra being thus enclosed. In Echidna (Fig. 403, B) one sees that the urethra is made up of two parts — an upper derived from the urogenital sinus, and a lower or penile from the channel enclosed by the lateral vestibular folds. At the junction of these two parts of the urethra there is still an orifice forming a communication between the urogenital sinus and the cloaca and representing the primitive opening of the urogenital sinus (Fig. 403, B). In marsupials the primary urethra (urogenital sinus) and secondary or penile urethra have united by the closure of the primitive opening of the urogenital sinus, and thus the passage for the urine and semen become completely separated from the passage for the faeces. The rectum is detached from the urogenital sinus and opens directly into the ectodermal cloaca.

600px Fig. 406. Case of a Female Child in which the Rectum opened on the Vestibule while the Normal Anus remains closed by the Anal Plate. The opening on the vestibule represents the ancient cloacal orifice of the rectum.


Differentiation of the Human Cloaca

We are now in a position to interpret the changes which occur in the human cloaca during the 5th, 6th and 7th weeks of development (Fig. 407). In the 5th week the rectum ends proximal to the Wolffian duct as in the frog ; in the 6th week the cloacal orifice of the rectum has moved backwards, leaving the proximal part of the cloaca as the urogenital sinus, a condition similar to that seen in Echidna (Fig. 405, C). As in that animal, the Wolffian ducts and ureters open close together in the sinus. The appearance presented by the backward migration of the rectal orifice is exactly the same as if the cloaca had been divided into rectal and urogenital cavities by the septum marked " A " in Fig. 407, B, C. It is convenient to apply the term given by Retterer to this septal formation — the urorectal septum. In the 7th week (Fig. 407, C) the orifice (cloacal anus) of the rectum reaches the cloacal depression (ectodermal cloaca) and thus become separate from the urogenital sinus — which now represents practically the whole of the entodermal cloaca. During the 6th week the ventral or pubic part of the entodermal cloaca is extended forwards to provide the lining of the penile urethra. It represents a direct extension of the urogenital sinus carried within the outgrowing genital eminence (Fig. 407, C). The cloacal membrane on the floor of this penile extension of the cloaca breaks down towards the end of the 7th week and the urogenital sinus thus presents a fissurelike opening on the perineum. This fissure corresponds to the groove on the open urethra of the tortoise (Fig. 403, A). On each side of the perineal fissure, towards the end of the 7th week there appears a fold — the vestibular fold or seminal guide. The hinder ends of the vestibular folds are continuous with the urorectal septum ; they unite together in the middle line, union commencing at the urorectal septum and spreading forwards. Their union closes the ancient cloacal orifice of the rectum, but cases frequently occur in which the closure is imperfect and the ancient cloacal anus persists (Figs. 406, 408, 409). It will thus be seen that in the human embryo the rectal orifice migrates backwards until it opens in the posterior part of the perineal depression (ectodermal cloaca), leaving the w^hole of the entodermal cloaca of the embryo as a urogenital passage or urethra. All these changes take place during the latter part of the 2nd month.


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Fig. 407. Showing the manner in which the Rectum becomes separated from the Urogenital Sinus (entodermal cloaca) during development of the Human Embryo. A, Human embryo of 5th week, 4 mm. long ; after Keibel. B, From human embryo of 6th week, 11 mm. long ; after Keibel. C and D, Later stages of development ; 1, bladder ; 2, Wolffian duct (ureter and vas) ; 3, entodermal cloaca ; 4, rectum ; 5, anus ; CM., cloacal membrane ; U.G., urogenital sinus ; A, Urorectal septum ; B, penis ; **, the limits of the perineal depression (ectodermal cloaca).

Malformations of the Rectum and Anus

When the rectum reaches the perineal depression, it is in contact with and closed by the cloacal membrane (Fig. 407, C). The union of the urorectal septum with the hinder ends of the vestibular folds gives rise to the perineal body which separates the anus from the vestibular cleft. The posterior part of the cloacal membrane proliferates, and forms the anal plug. The plug breaks down at the commencement of the 8th week, and the permanent anus is thus formed. This process may fail, giving rise to the condition known as atresia ani or imperforate anus. A common degree of malformation is shown in Fig. 408. The migration of the rectum has failed ; it opens into the urethra by the ancient cloacal anus and a thick stratum of mesodermal tissue separates the rectum from the anal depression formed by the ectodermal plug derived from the hinder end of the ectodermal cloaca. In Fig. 409 an exactly similar condition is represented in a female infant. The rectum opens in the male below the orifice of the uterus mascubnus, in the female at a corresponding point below the orifice of the vagina. The urorectal septum and vestibular folds in the female form merely the perineal body, which separates the terminal part of the rectum from the vulva ; in the male they form the floor of the urethra and perineum from the sinus pocularis to the lacuna magna in the glans penis. The terminal part of the male urethra, as we shall see presently, has a separate origin. The downward migration of the vaginal orifice in women brings the cloacal opening of the rectum into the vulva — ^the floor of the vulval cleft being a derivative of the urogenital sinus. In many cases of imperforate anus (Fig. 410) the cloaca! anus is closed, and tte rectum terminates an inch or more from the anal depression. In other cases merely a thin septum separates the anal depression from the termination of the rectum. In extreme cases, which are by no means rare, no anal depression is formed and the sacral part of the rectum is absent.

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Fig. 408. Section of Pelvis of a Male Child, showing the Rectum ending in the Prostatic Part of the Urethra. J., bladder; 5, rectum; C, recto-vesical pouch ; D, uterus masculuius ; E, intra-cloacal anus ; F, prostate ; G, anal depression (ectodermal) ; H, external and internal sphincters ; /, Cowper's gland.


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Fig. 409. Section of Pelvis of Female Infant, shoM'ing the Rectum opening into the Navicular Fossa of the Vulva. A, bladder ; B, rectum ; C, recto-uterine fold ; D, symphysis ; E, vulval anus ; F, cervix ; G, anal depression (rarely present if rectum opens in vulva) ; E, urethra ; /, clitoris ; K, hymen ; L, Labium minus.

There is some confusion as to how much of the anal canal is formed from rectum and how much from anal depression. As may be seen from Figs. 408 and 409 certain folds are formed in the wall of the anal depression. At the upper end of these anal folds, which are scarcely recognizable in the fully developed anal canal, are develoj)ed certain valve-like folds of mucous membrane, the anal valves. Above the valves are the well-marked columns of Morgagni formed in the rectum. The valves mark the junction of the rectum with the anal depression. ^


Fig. 410. A Case of Imperforate Anus in which the Rectum has been arrested in its migration from the Cloaca to the Perineum.


' See research by F. P. Johnson, Amer. Journ. Anat. 1914, vol. 16, p. 1.


External Genital Organs and Perineum

That the interpretation just given of the embryological parts entering into the formation of the rectum and urethra is right is seen when the development of the external genital organs is traced. The stages in the development of the human urethra, penis and scrotum during the latter part of the 2nd month and earlier part of the 3rd are shown in Fig. 411. Stage I. represents the condition seen in the perineum about the end of the 8th week. The circular fold A — cloacal fold ^ it may be named, for it represents the opening or margin of the primitive (ectodermal) cloaca. Within its anterior or pubic fornix is rising up the genital eminence to form the penis or clitoris — according to sex, for at this time the external parts of both sexes are alike, although the ovary is being differentiated from the testicle. There is a groove or furrow on the under or cloacal aspect of the genital eminence, as on the cloacal penis of the tortoise ; it represents the roof of the penile urethra, and leads backwards to the urogenital sinus. The folds at each side of the furrow {a, a) are the vestibular or perineal folds which form the penile urethra in Echidna. In Stage II. (Fig. 411), reached during the 9th week, two further changes are seen in progress. The lateral perineal folds {a, a) have now united behind the genital or urethral furrow, and by so doing have separated the anal part of the ectodermal cloaca (perineal depression), in which the rectum now terminates, from the anterior urogenital part. By the union of the posterior ends of those lateral perineal folds the perineal body is formed. The cloacal folds {A, A) are still well marked, but it is apparent that the genital eminence and its attached folds are being extruded from the cloaca. In Stage III., reached at the commencement of the 3rd month, a condition is represented which is common to both male and female foetuses. The anus is now extruded from its depression, and lies within the flattened posterior fornix of the cloacal folds. The lateral perineal or vestibular folds meet behind in the perineal body, where their free margin forms a semilunar fold — the primitive fourchette. Anteriorly the folds unite on the perineal aspect of the glans. Between the folds opens the penile urethra ; the opening between the folds is the orifice of the urogenital sinus ; it represents the primitive meatus of the penile urethra. On the under or perineal aspect of the glans a depression (the phallic groove) packed with an ectodermal plug is also present ; it forms the part of the urethra within the glans. Stage IV. represents a condition peculiar to the male. A median raphe or suture is now seen extending from the anus behind to terminate in front in the two perineal or vestibular folds — perhaps it would be well to name their anterior parts, which are confined purely to the urethra of the male and vestibule of the female — urethral or inner genital folds. The primitive urethra is now small in size and well advanced towards the glans penis. The median perineal raphe is caused by a fusion of the tissues of the cloacal or outer genital folds within the septum primarily formed by the union of the lateral perineal folds. In the female this union of the cloacal folds does not occur, and hence there is no raphe on the female perineal body (Wood Jones). The cloacal folds remain separate, and form the labia majora ; in the male they unite, and form the scrotum.


^ Usually named the outer genital fold.



Fig. 411. Stages in the development of the Human Penis and Perineum. (Drawings by Dr. Stanley Beale after figures given by Kollmann, Keibel and Hertzog.) I." human embryo 25 mm. long (8 weeks) ; II. 29 mm. long (9th week) ; III. 31 mm. long (9th week) ; IV. 45 mm. long (about 10th week). A, lips of cloaca (labit majora) ; B urogenital orifice being carried to the surface between labia minora (a, a) ; C, penis becoming extra-cloacal ; D, Tail ; E, urogenital orifice ; F, anus ; G, meatus.


By the end of the 3rd month the process of union which gives rise to the perineal raphe extends to the glans, and in this way the primitive meatus is closed, the terminal parts of the vestibular folds forming the frenum of the prepuce. Thus the anterior parts of the perineal folds imite right up to the fraenum of the prepuce, and enclose the male urethra. In Stage IV. (Fig. 411) the margins of the phallic groove on the glans have united ; the plug of epithelium within it breaks down as it unites with the cloacal urethra, and the permanent terminal urethra and meatus are thus established. In Stage IV. the preputial collar of skin is seen in process of formation. It is directly continuous with the anterior ends of the folds surrounding the primary meatus. It does not rise up as a free fold ; ^ the epithelium on the deep surface of the collar adheres to that on the glans, and hence when the prepuce is fully formed in the 4th month, the prepuce is tightly bound to it until the period of birth.

1 Dr. Berry Hart {J own. Anat. and Physiol. 1908, vol. 42, p. 50) and Dr. Wood Jones {Brit. Med. Journ. 1910, Jan. 15th) give another interpretation of the manner in which the prepuce is formed.


Hypospadias

It is not unusual to see cases in which the process of urethral development has been arrested. In the female its complete arrest is normal ; in individuals with imperfect differentiation of sexual glands (usually imperfect males) the process is also arrested at an early stage. Fig. 412 represents three conditions of hypospadias due to arrest of development at the terminal stage. In A the phallic groove is unclosed ; the urethra opens at the primary meatus ; the folds bounding the meatus represent the anterior ends of the urethral or perineal folds. In B the primitive meatus is unclosed, but the phallic groove is converted into a canal ; in C, the commonest type, the primitive meatus is open and the phallic groove has remained uncanaliculized.^



Fig. 412. Three types of Hypospadias — A, in which the Groove in the Glans (phallic groove) is open, and Urine passes by the primitive meatus ; B, in which the floor of the Phallic Groove is formed, but the primitive meatus is unclosed ; C, in which the Phallic Groove is unformed or obliterated and the primitive meatus persists. 1, Primitive meatus ; 2, fraenum praeputii ; 3, phallic groove and canal ; 4, permanent meatus.


Fig. 413. Stages in the Evolution of the Perineal Musculature. (After Popowsky.) A, sphincter of the cloaca in the 2nd month ; B, its division at the beginning of the 3rd month ; C, its condition in the male foetus at the end of the 3rd month.

Perineal Muscles

From what has been said regarding the cloaca, the evolution of the muscles of the perineum from the sphincter of the cloaca will be readily understood. The sphincter in cloaca! vertebrates surrounds the part of the cloaca (perineal depression) formed from ectoderm ; it is a striated muscle. At the end of the 2nd month this muscle is apparent in the cloacal fold of the human foetus (Fig. 413, A). At the beginning of the 3rd month, when the perineal body is formed, the sphincter of the cloaca becomes divided into urogenital and anal parts ^ (Fig. 413, B). The sphincters of the urogenital passage and anus fuse in the perineal body. A part of the urogenital sphincter obtains an attachment to the ischium and forms the ischio-cavernosus (erector penis) ; another strand, the transversus perinei. With the formation of the urethra in the male, the sphincter of the urogenital passage is carried forwards on the bulb and forms the bulbo-cavernosus ; in the female it remains as the sphincter vaginae. A deeper and older part surrounds the upper part of the urogenital sinus, and becomes the constrictor urethrae.


i See Ralph Thompson, Jo^lrn. Ayiat. 1919, vol, 53, p. 32,



Origin of the Bladder

In amphibians the bladder is a diverticulum of the cloaca. In the embryos of reptiles, birds and mammals it becomes modified, to form the allantois ; part lies outside the body and is lost at birth, part remains within the body to form the urachus and all the bladder except the trigone. By a downward migration of the orifices of the Wolffian and Mlillerian ducts, the upper part of the urogenital sinus, containing the insertion of the ureters, remains to form the trigone of the bladder and supra-genital part of the urethra (Figs. 388, 407).


The Urachus

When the body stalk becomes elongated in the formation of the umbilical cord, the part of the allantoic cavity within it is obliterated. The part of the allantois within the abdomen, stretching from the umbilicus to the apex of the bladder, becomes the urachus, a fibrous cord, in which all trace of the allantoic cavity is lost (Fig. 416). Occasionally traces of the cavity may remain and form urachal cysts,^ or it may remain open throughout, so that urine escapes from the bladder by a fistula at the umbilicus. The urachus lies behind the linea alba, embedded in the subperitoneal tissue, and flanked on each side by the umbilical artery. In the 7th month the apical part of the bladder is attached by a mesentery to the anterior wall of the abdomen ; later the mesentery disappears (Broman).


The Bladder at Birth

At birth the bladder is elongated and fusiform in shape, with a small trigone (Fig. 416). The capacity of the pelvis is relatively less than in the adult ; hence the greater part of the bladder is supra-pubic in position.


Ectopia vesicae ^ is not easily explained on an embryological basis. The condition is shown diagrammatically in Fig. 414, A. The anterior wall of the bladder and roof of the urethra and the parts of the belly wall in front of these are absent ; the trigone, floor of the urethra, and posterior wall of the bladder are flush and continuous with the belly wall. The symphysis pubis is not formed. Certain chambers in the embryo, such as the neural canal and pericardium, are liable to a dropsy and rupture. Were the cloaca of the embryo to become ruptured along its ventral wall (Fig. 414, B) the condition of ectopia would be produced. Further, it is to be remembered, as Berry Hart lias pointed out, that the part of the embryo on which the primitive streak is sitviated comes to form the postumbilical part of the ventral wall of the abdomen (see page 37). It is therefore more probable that the condition may be due to an unclosed condition of the primitive streak.


1 W. J. Otis, Anat. Hefte, 1905, vol. 30, p. 199 (Dev. of Anus and External Sphincter).

2 See Alban Doran, Proc. Boy. Soc. Med. April, 1908.

^ For current theories see Wood Jones, Journ. Anat. and Physiol. 1912, vol. 46, p. 193 ; Keith, Brit. Med. Journ. 1908, Dec. 26th ; A. M. Paterson and Emrys-Roberts, Journ. Anat. and Physiol. 1906, vol. 40, p. 332,


Musculature of the Bladder, Urethra and Rectum

Seeing that the rectum, allantois and cloaca are continuous in the embryo one would expect that the musculature of the parts should show traces of this continuity. Mr. F. Wood Jones found (1) that the band of musculature which passes from the front of the rectum to be lost in the tissue behind the membranous urethra is a remnant of the recto-cloacal communication in the embryo (Fig. 408) ; (2) that the circular muscular coat of the urethra is continuous above with the sphincter and circular coat of the bladder, and below it becomes continuous with the striated fibres (constrictor urethrae) surrounding the membranous urethra. The latter, however, are not developed from the musculature of the urogenital sinus, but from the sphincter cloacae (Fig. 413).


Fig. 414, A. A Section to show the condition of parts of Ectopia Vesicae. B. Section of the Pelvis of an Embryo (5th weeli) to sliow how the condition is probably produced.


Neurenteric Canal

Ano-coccygeal tumours are believed to arise from remnants of the neurenteric canal as well as from the post-anal gut. The neurenteric canal, or blastopore, it will be remembered (p. 38), is a communication of the cavity of the archenteron with the dorsal surface of the embryo. The blastopore opens at the anterior end of the primitive streak, which afterwards is included in the posterior end of the neural groove ; such a canal might be represented by a remnant passing from the rectum to the sacral region of the spinal canal. A vestigial structure, which is certainly of this nature, has not so far been recognized. The part of the hind-gut which is developed under the tail of the embryo (post-anal gut) disappears in the 2nd month of development, but certain congenital tumours which arise between the sacrum and coccyx may spring from remnants of the post-anal gut (p. 381). Seeing that the embryonic tissue which gives rise to the caudal end of the body lies in the posterior lip of the blastopore, it is not difficult to conceive that embryological remnants might persist in the sacro-coccygeal region of the pelvis and give rise to teratomatous tumours.


External Genitals of the Female

In the female the parts retain closely the early foetal form represented in Fig. 415. The genital tubercle' becomes the glans clitoris. In the genital eminence — of which the tubercle is merely the summit — ^the corpora cavernosa develop. The vestibular or inner genital folds form the labia minora, the prepuce and fraenum. By the junction of the inner genital folds within the urogenital depression behind, the fourchette is formed. Thus the opening into the urogenital sinus (primitive meatus) is bounded by the fourchette, labia minora and fraenum of the prepuce. In the lateral folds, or labia minora, are developed the bulbs of the vestibule. After the third month the external genital (cloacal) folds become prominent around the urogenital depression and form the labia majora. By their anterior union they give rise to the mons veneris. Their posterior extremities fade away posteriorly (Fig. 411). After the 3rd month the external genital parts undergo a change directly opposite to that which takes place in the male. The primary meatus and penile urethra open up and form the vestibule, into which open urethra and vagina. This change is probably due to the influence of the ovarian germinal tissue.


Fig. 415. Diagram showing the terms usually applied to the External Genital Parts of the Embryo. The outer genital represent the cloacal folds ; the inner genital folds the anterior parts of the vestibular folds ; the urogenital depression or cleft, the primary meatus (see Fig. 411).


External Genitals of the Male

In the male, at the end of the 2nd month, the inner genital folds corresponding to the fourchette and labia minora, grow forwards as a crescentic shelf, thus closing the urogenital cleft and forming the floor of the penile urethra. While the floor of the penile urethra is formed thus, its roof, corresponding to the vestibule of the female, is derived from the urethral or forward prolongation of the cloaca (see Fig. 407). The erectile tissue in the inner genital folds, which forms the bulbs of the vestibule in the female, forms the corpus spongiosum in the male. The corpora cavernosa are formed m the genital eminence. The anterior part of the corpus spongiosum is formed separately in the apical part (glans) of the genital eminence. The corpora cavernosa are developed by the enlargement of capillary vessels of the body of the genital eminence during the 4th month. The part of the urethra within the glans is the last j)art to be formed, and its development, as we have seen, is peculiar (p. 389). The part of the urethra within the glans becomes canaliculized a short time before birth. TI13 fossa navicularis and lacuna magna occur at the junction of the part of the urethra formed in the glans and the part formed from the urogenital sinus. ^

The scrotum is formed during the 3rd month by the union of the external genital folds (labia majora of the female), the raphe formed by their union extending from the fraenum of the prepuce in front to the anterior margin of the anus behind (see p. 388).



Fig. 416. A Section of the Male Bladder and Urethra at Birth, showing the derivation of parts.


The Prostate

The prostate is developed by outgrowths of the entodermal lining the upper part of the urogenital sinus and from the mesodermal tissue surrounding the sinus. It consists of glandular tissue and stroma.

(1) The glandular tissue is composed of tubular glands which open into the prostatic part of the urethra. They are developed in the 4th month, as a series of solid buds, about 60 in number, from the epithelium lining the upper part of the urogenital sinus (Fig. 417). The buds, which soon become tubular in form, arise from a right and left longitudinal furrow or fold of the wall of the sinus between which the Wolffian ducts open (Pallin).



^ For literature on development of urethra see A. Lichtenberg, Anat. Hefte, 1906, vol. 31, p. 63 ; J. E. Spicer, Journ. Anat. mid Physiol. 1909, vol. 43, p. 195.

^ For papers on the development of the prostate see E. J. Evatt, Journ, Anat, and Physiol. 1909, vol. 43, p, 314 ; 1911, vol, 45, p. 122.


The prostatic furrows reach upwards above the Wolffian openings into the purely urinary part of the sinus and downwards into the part which serves as a common passage for the semen and urine. These segments of the sinus become the upper and lower parts of the prostatic urethra. The buds from the right and left furrows form two lateral masses or lobes. At first the two lateral lobes, as in mammals generally, He separately behind the urethra. Other outgrowths also arise from the anterior or pubic side of the sinus — some of these afterwards undergo atrophy — from the side or lateral aspect of the sinus (Fig. 418). The lateral prostatic masses fuse behind the urethra ; in man only do they meet to form an anterior or pubic commissure over it. The tubules of the median or third part arise from the middle line of the floor of the sinus above the openings of the Wolffian ducts (ejaculatory ducts) (Evatt), but the lateral lobes also fuse with this median element, and help to form it. It may be small or even absent.^ 1 J. W. Thomson Walker, Journ. Anat. and Physiol. 1906, vol. 40, p. 189 ; 0. S, Lowsley, Amer. Journ. Anat. 1912, vol. 13, p. 299.


Fig. 417. A Diagram to show the position at which the Prostatic Tubules arise.


Fig. 418. The Prostate and Urethra towards the end of the 4th month. (After Broman and Evatt.) The phallic part of the urethra ends posteriorly at the lacuna magna. It is developed in the glans. The uterus masculinus (ut. mas.) is indicated diagrammatically to show its relationship to the common ejaculatory duct.


Skene's tubules, which may be found opening into the urethra of the female, represent prostatic tubules. A reference to Figs. 408, 409 will show that the female urethra corresponds to the upper j^rostatic urethra of the male, and that the floor of the vestibule — in which rudiments of prostatic tubules may be formed — represents the lower prostatic urethra.


(2) The Stroma of the Prostate. — While the glandular tubes arise in linear groups from the epithelium lining the urogenital sinus — the muscular and fibrous elements arise from the mesodermal tissue of the genital cord in which the terminal parts of the Wolffian and Miillerian ducts are situated and from the circular musculature of the urogenital sinus (see Fig. 419). When the glandular elements grow out they become embedded in and carry before them the circular musculature of the urogenital sinus which thus forms the muscular cortex or inner capsule of the prostate. Probably the stroma of the genital cord also contributes to the musculature of the prostate. The musculature of the uterus, which is also developed from the genital cord, like that of the prostate, is liable to become the seat of fibromyomatous growths in the later years of adult life. As regards the nature of the prostate : (1) It is purely genital, and develops only in the rutting season in such mammals as manifest a seasonal sexual life. Its development in the female is arrested at a very early stage — probably the result of an ovarian influence.


Fig. 419. Diagrammatic Section of the Bladder and Urethra of a 6th month Foetus to show (1) the development of the Prostate, (2) the relationship of the Prostatic Musculature to that of the Uretlira and Bladder. (Wood Jones.)


(2) It remains comparatively undeveloped until puberty. At the age of seven it weighs only 30 grains ; after sexual life is established it weighs about 300 grains.

(3) The healthy prostate atrophies if castration is performed, but this operation has no efiect on glands which have become pathologically hypertrophied.i In one man out of three over 55 years of age the prostate hypertrophies, both the glandular and fibro-muscular elements participating. Hypertrophy of the median part causes a valvular elevation behind the vesical opening of the urethra.


Glands of Cowper and Bartholin are produced as solid buds from the entodermal lining of the penile extension of the urogenital sinus (Fig. 418). Hence in the female the ducts of Bartholin's glands open in the vulval cleft just outside the hymen at each side of the vagina, for the hymen marks the junction of the Miillerian ducts with the urogenital sinus. In the male the ducts of Cowper's glands open in the bulbous part of the urethra (Fig. 418). Their function is unknown, but they are certainly sexual in nature. The numerous glands of Littre, like Cowper's and Bartholin's glands, are produced by tubular outgrowths during the fourth month (Fig. 418). In the male the glands of Littre are produced most numerously along the dorsal aspect of the urethra.


Round the anus, and especially in the furrow between the labium minus and majus, groups of large peculiar sebaceous glands are produced in the 4th and 5th months, corresponding to the anal and preputial glands of mammals. Occasionally two groups of sebaceous glands occur on the prepuce of the male, especially if hypospadias be present (Shillitoe). Glands round the corona of the glans penis (Tyson's glands) are only very exceptionally present.


The Testes

Descent and Development of the Testicle

The origin of the testis on the inner or mesial side of the Wolffian ridge, and its attachment to the dorsal wall of the coelom by a mesentery common to it and the Wolffian body have been already described (see Figs. 4, 23, 384). The position of the testicle in a foetus of the third month is shown in Fig. 420. Although m the 6th week the genital ridge extended from the 6th to the 12th thoracic segment, the testicle, developed from the hinder part of the ridge, is now situated in the iliac fossa. The mesorchium, a fold of peritoneum, binds its attached border to the iliac fossa. At its outer side lies the genital part of the Wolffian body which forms the epididymis. It, also, is suspended by a mesentery — -the Wolffian mesentery. The two mesenteries have a common base — the common urogenital mesentery (see Fig. 384). The upper part of the urogenital mesentery forms the diaphragmatic fold, with which the peritoneal fold containing the spermatic artery becomes joined ; to the combined fold is given the name of plica vascularis (compare Figs. 421, 424). This in the female becomes the ovario-pelvic ligament (Fig. 5). A fold of peritoneum, the inguinal fold or plica gubernatrix, continues the common urogenital mesentery to the groin (Fig. 420). The gubernaculum testis is developed in the plica gubernatrix ; in the corresponding fold in the female the round ligament of the uterus appears (see p. 371). The vas deferens (Wolffian duct) turns into the pelvis from the lower end of the epididymis (Wolffian body), and within the pelvis lies in the genital cord (Fig. 392). A remnant of the Miillerian duct lies along the inner and ventral aspect of the epididymis.


1 E. Pittard, Convpt. Rend. Acad. Sc. 1911, vol. 152, p. 1617 (Effect of Castration) ; Cuthbert S. Wallace, Trans. Path. Soc. Lond. 190^r vol. 56, p. 80 ; W. L. H. Duckworth, Journ. Anat. and Physiol. 1906, vol. 41, p. 30 (Eunuchoid Man) ; R. H. Whitehead, Anat. Bee. 1908, vol. 2, p. 177, A7ner. Journ. Anat. 1905, vol. 4, p. 193 (Dev. and Nature of Interstitial Cells).

2 See Eben C. Hill, Amer. Journ. Anat. 1907, vol. 6, p. 439 (Dev. of Blood Supply) ; D, T. Barry, Journ. Anat. and Physiol. 1910, vol. 44, p. 137 (Differentiation of Tubules),


Seminiferous Tubules

The arteries for the genital glands represent the lowest of the vessels which originally supply the Wolffian body and arise from the aorta at the level of the 12th dorsal vertebra ; their nerve supply comes from the 10th dorsal segment of the spinal cord. The testis is therefore developed in the genital ridge between the 10th and 12th dorsal segments. The early development of the testis is similar to that of the ovary. Up to the 7th week, when the embryo measures 15 mm. in length, it is impossible to tell testicle from ovary ; both of them at this time show a covering of germinal epithelium and deep central masses or columns of epithelioid cells derived from the covering layer of germinal epithelium. In the central masses are the large primitive germinal cells (primordial ova). At the end of the 7th week two changes lead to the difierentiation of a testis from an ovary ; (1) a tunica albuginea begins to form under the superficial epithelium, (2) the central masses proliferate and form radiating cords which branch and anastomose as they spread from hiluni to periphery. The cords become transformed into the seminiferous tubules which are at first solid. Some of the epithelioid cells are not included in the tubes and remain to form interstitial cells.'^ The genitaloid cells are included in the epithelial cords. The tubules become separated into groups or compartments in the 6th month and about the same time lumina are formed in them. The formation of spermatozoa has been already described (p. 9). The visceral layer of the tunica vaginalis on the testicle represents the covering of flattened epithelium which remains after the ingrowth of the germinal epithelium. The vasa efferentia and coni vasculosi are formed from the genital Wolffian tubules ; the rete testes and vasa recti from the junctional cords (p. 361). Into the rete open the tubuli seminiferi formed in the testicle. The epididymis is the elongated upper segment of the Wolffian duct (Fig. 386). The Wolffian elements are produced within the Wolffian ridge (Fig. 390).


1 See articles by Sir F. W. Mott, Brit. Med. Journ. 1919, vol. 2, p. 655 ; T. Russell Godclaid, Journ. Aval. 1920, vol. 54, p. 173 ; B. F. Kingsbury, Amer. Journ. Anat. 1914, vol, 16, p. 59.


Fig. 420. The Position of the Testis in a Foetus of 2i months.


Formation of the Gubernaculum Testis

There is no trace of the inguinal canal in the 3rd month of foetal life ; the various layers of abdominal wall are unbroken, except for a fine strand of tissue which leads towards the site of the scrotum, and evidently serves as a guide for the gubernacular outgrowth. In the fourth month the subperitoneal layer of non-striated muscular tissue in the plica gubernatrix and in the mesorchium takes on a rapid growth (Fig. 421). At the same time the tissues of the abdominal wall undergo a localized evagination towards the scrotum. They are probably carried down by the growth of the gubernacular bud which pushes its way to the scrotum. The gubernaculum grows downwards as a solid cellular mass, until it reaches the subcutaneous tissue which at that time completely fills the scrotum. Its attachment to the scrotum is sHght and easily broken (Fig. 421). The gubernaculum, as it grows through the abdominal wall, carries with it : (1) A process of peritoneum (the processus vaginalis) ; (2) The transversalis fascia (the infundibuliform fascia) ; (3) The internal oblique and transversalis muscles to form the cremaster ; (4) The spermatic fascia from the external oblique ; (5) The deep layer (Scarpa's) of the superficial fascia of the groin. All these layers are added to the primitive coverings of the scrotum, which until then is made up simply of skin and superficial fascia (Fig. 421).


1 See John Hunter's classical account, Palmer's Edition of his Works, vol. 4, 1837. Also paper by Dr. Berry Hart, Trans. Edin. Obstet. Soc. 1909, vol. 34, p. 151.


Fig. 421. Showing the Position of the Testis at the 6th month, and the Formation of the Gubernaculum Testis.


It will thus be seen that the gubernaculum testis is a fibro-muscular mass with an actively growing cellular cap, which, starting from the muscular stratum in the mesorchium and plica gubernatrix in the iliac fossa, invades the abdominal wall, every layer of which it carries as a prolongation within the scrotum. It is an invading army of cells. It draws with it into the scrotum the peritoneum in the iliac fossa, on which the testis is dragged like a log on a sledge.

The testis descends from the loins to the iliac fossa in the 3rd month ; from the 4th to the 7th month it rests at the site of the internal ring ; it spends the 7th month of foetal life in its exodus through the abdominal wall. In the 8th month it leaves the inguinal canal and lies at the external abdominal ring. After birth it reaches the fundus of the scrotum. The atrophy and contraction of the gubernaculum pull it down. A remnant of the gubernaculum can always be found in the adult behind the epididymis and testicle, within the mesorchium (Fig. 423).


Fig. 422. The manner in which the Structures in the Wall of the Abdomen are carried out so as to form the Inguinal Canal and Coverings of the Testis.


Processus Vaginalis

The processus vaginalis becomes occluded by adhesion or zygosis (p. 287) at two points soon after birth, but in a considerable proportion of individuals the process of closure is delayed (Fig. 423). The upper point of occlusion takes place at the internal abdominal ring ; the lower a short distance above the testicle. The part of the processus vaginalis between the points of occlusion is known as the funicular process ; the part surrounding the testicle becomes the tunica vaginalis. In quite 30 % of children the occlusion takes place at the internal abdominal ring some considerable time after birth or it fails to appear altogether. Occlusion may fail at the upper pointy at the lower point, or at both. Or it may close at both points, but the funicular process, instead of disappearing, may remain open and form a cyst.


Descent of the testicle ^ may be arrested at any stage ; often in the inguinal canal ; more frequently at the external abdominal ring. Arrest of descent is commonly regarded as a symptom of arrest of testicular development. John Hunter regarded arrested descent of the testicle as due to an imperfection in its development ; all recent observations support his opinion. There can be no doubt that in all those mammals ^ in which the testis leaves the abdomen it does so to escape the intra-abdominal pressure to which the abdominal viscera are subjected. Its descent is correlated with the evolution of the diaphragm and exclusion of the lungs from the abdominal cavity. Violent respiration and flexure of the trunk give rise to very high degrees of tension within the abdomen ; from some cause at present not understood, a testicle atrophies when subjected to this pressure. On the other hand, the testicle may assume an ectopic position. The gubernaculum, as it makes its way towards the scrotum, may take an eccentric course, and bring the testicle to rest in the groin, root of the penis, or over the pubis.


Fig. 423. A Diagram of the Processus Vaginalis.


Mesorchium

The testis and epididymis were suspended within the abdomen by the common urogenital mesentery (Fig. 384). In the course, of the descent of the testis this becomes shortened by the development of the gubernaculum, and the testis and epididymis become thus firmly bound by their posterior borders to the tunica vaginalis. The digital fossa, situated between the mesorchium and mesentery of the Wolffian body, represents the recess which separated the genital from the Wolffian ridge of the embryo. The mesorchium — the true mesentery of the testicle — may assume the form of an elongated fold, attaching the testicle to the epididymis (Corner).

^ D. Berry Hart, Journ. Anat. and Physiol. 1910, vol. 44, p. 4.

2 See W. N. F. Woodland, Proc. Zool. Soc. London, 1903, vol. 1, p. 319.


A not unusual anomaly of the testicle is represented in Fig. 425. It will be observed that the common mesentery, in place of becoming shortened, and thus fixing the testicle and epididymis widely to the peritoneum, becomes narrow and elongated. Such testicles are usually arrested in their descent, and are apt to twist and become strangulated. It will also be observed that a gubernaculum is present, but it has seized and drawn downwards only a loop of the vas deferens. The explanation is shown in Fig. 424. The inguinal fold is made up of two parts, a lower, ending on the vas deferens and corresponding to the round ligament of the female ; an upper, which continues the fold to the epididymis and testicle, and which corresponds to the round ligament of the ovary. In such cases, then, the gubernaculum has not extended to the upper part of the inguinal fold.



Fig. 424. To show the Diaphragmatic Fold (upper part of the common genital mesentery), Vascular Fold, and the two parts of the Inguinal Fold in a Foetal Pig. (Eben. C. Hill.)


Fig. 425. Elongated Common Mesentery of a Testicle arrested in the course of its descent.


Hermaphrodites

Online Editor - Please note this is a historic terminology, the current term is Disorders of Sex Development.

A hermaphrodite — a human individual in which both testis and ovary are present — has never been seen. Dr. Bulloch found only five cases on record in which, within the same genital gland, there were present representations of imperfect testicular and ovarian tissues (ovario-testis) ; spermatozoa and ova were not present. The term is usually applied to individuals in whom the genital glands are imperfectly developed. Usually they are imperfect males. It is clear that sexual differentiation commences in the 7th week (although the' sex is probably determined at, or even before the time of fertilization) ; by some means probably by an internal secretion — the ovarian and testicular tissues exercise a direct and opposite influence on tbe development of genital structures. Hence, if the gland is imperfect, development of tbe genital parts is uncontrolled and represents a condition which may best be described as neutral. There is evidence to support the opinion that the embryonic genital gland is composite ; the testicle develops within the medullary or central part of the gland ; the ovary from its cortical or more superficial parts.


^ See Bulloch, Treasury of Human hiheritance, London, 1909, Part 3, Section Xa ; Berry Hart, Proc. Roy. Soc. Edin. 1909, vol. 29, p. 607, 1910, vol. 30, p. 230 ; J. F. Gudernatsch, Amer. Journ. Anat. 1911, vol. 11, p. 267 ; F. R. Lillie, Journ. Experim. Zool. 1917, vol. 23, p. 371.


Development of the Suprarenal Bodies

The suprarenal or adrenal bodies arise by the association of two distinct embryological elements • — cortical and medullary. In Fig. 427 is given a diagrammatic section to show the stage reached in the 7th week of development, the cortical element then being large and projecting at the root of the mesentery and in contact with the genital and Wolffian bodies, while the medullary element consists of indifferent cells connected with the blastema of the sympathetic system. The cortical element is associated with the genital system ; its secretion regulates the development of certain sexual structures and functions. Like the epithelioid cells of the genital glands, the cortex arises by an invagination of coelomic epithelium, the suprarenal ingrowth occurring on each side of the root of the mesentery early in the 6th week. The suprarenals are developed within the anterior 'ends of the Wolffian bodies, just behind the pleuro-peritoneal passages. The medulla arises from groups of cells which also form sympathetic ganglia ; probably from the primitive cell basis of the semilunar ganglion, which is developed by the side of the aorta, close to the pleuro-peritoneal opening. Hence the great plexus of nerves which passes from the solar plexus to the medulla of the suprarenals. The medullary cells begin to migrate into the cortex in the 8th week ; the process of invasion is continued through the greater part of foetal life. The invading cells, when stationed in the cortex, give rise to broods of chromogenic cells and later to sympathetic nerve cells.


Fig. 426. Showing the distribution of the aortic chromaffin bodies in the early human foetus. (After Zuckerkandl.)


Fig. 427. Section across the suprarenal, genital and Wolffian bodies in an embryo in the 7th week, 15 mm. long. (After Zuckerkandl.)


By the beginning of the 4th month they are arranged aS reticulating columns set on the walls of branching venous sinuses.

The cortical cells range themselves in rows between radiating blood sinuses. As the kidneys ascend in the 3rd month they come in contact with the suprarenal bodies. The suprarenal is at first larger than the kidney, even at birth they are nearly equal in size. The nerves and arteries enter the bodies on their renal surface ; the veins emerge on their anterior surface.


Until the 3rd month the suprarenal bodies are in contact with the upper pole of the testis or ovary. As the genital glands descend, the diaphragmatic fold is drawn from the suprarenal region and frequently carries with it buds of suprarenal tissue both cortical and medullary. It is therefore readily understood how isolated parts of the suprarenal body (accessory suprarenals) may occur in the broad ligament or in the spermatic cord above the testicle. Such accessory bodies are probably derived from the cortical element which is developed within the Wolffian ridge and body. With the descent of the ovary and testicle, which bring with them the Wolffian body, adjacent accessory suprarenals, if such be present, are also brought down, and may occasionally give rise to peculiar tumours.

Chromaffin Cells

The medullary part of the suprarenals belongs to a segmental series of organs. In such fishes as the shark and lamprey, a group of cells (a paraganglion) is thrown off from each ganglion of the sympathetic chain and comes into close contact with the tributaries of the cardinal veins. These cells stain brown with salts of chromium — hence their name ; some of these cells remain within the sympathetic ganglia. Similar minute chromaffin bodies (paraganglia) are also developed in or near all the ganglia of the vertebral chain of the human foetus. The carotid body arises in association with the upper cervical ganglion. Other collections of chromaffin cells arise at the sites of the prevertebral ganglia and plexuses — such as the superior and inferior mesenteric plexuses. The distribution of the aortic chromaffin bodies is shown in Fig. 426. Although chromaffin cells arise from the blastema of the sympathetic system yet they are differentiated before the nerve cells of that system, as if they represented the products of an earlier evolution. By their secretion they assist, or serve as substitutes for, the vasomotor sympathetic cells and for all nerve cells which have to do with regulating the action of nonstriated muscle. The medulla of the suprarenal represents the brain of the chromaffin system, but why it should be associated with a cortical element has not yet received an explanation.


Coccygeal Body is a small mass of chromaffin tissue, with rich blood supply, situated on the ventral aspect of the coccyx.

^ For an account of chromaffin tissue see an article by Swale Vincent, Journ. Anat. and Physiol. 1904, vol. 38, p. 34 ; E. Zuckerkandl, Keibd and MaWs Manual of Human Embryology, vol. 2, 1912.

2 J. Thomson Walker, ArcMv. fiir 3Iil: Anat. und Enlwkld. 1904, vol. 04, p. 121 (Coccygeal Body).





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Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures