406
ANATOMY
chromosomes. The act of fertilization consists in the reception by the ovum of certain nuclear masses, including chromosomatic substance, which unites with the nuclear substance and chromosomes of the ovum to form a new complete nucleus. The testis or male sex gland is also developed from the germinal area of the coelom. Nests of cells derived from the germinal epithelium become clustered and surrounded with a stroma of mesoblast into which the Wolffian tubules penetrate, ultimately becoming a tubular system, whose terminal roots enclose the cell nests as a lining Male epithelium. When first formed the testes lie in the lumbar region of the foetal abdomen above and internal to the kidneys, but from this site they begin to migrate about four months before birth. They reach the abdominal wall in the groin in two months, and there each passes through an oblique canal (the inguinal canal) in the muscular and fibrous wall of the abdomen and descends into the scrotum, which is a loose bag of skin formed by the lateral lips of the urogenital sinus (which long before this period have united medially). This descent is generally completed a month before birth. Along with the testis a tubular pouch of the serous lining of the abdomen is prolonged into the scrotum, which, when the descent is complete, becomes obliterated at the place where it traverses the abdominal wall, though remaining below as an envelope of the testis. The region of the inguinal canal is always the weakest spot in the abdominal wall, and it is along the track of the descended testis that those protrusions of the abdominal viscera, called hernice, are most liable to occur. As it is the assumption of the upright position which places the inguinal canal at the lowest part of the abdomen, this tendency to hernia is another of the penalties which man pays for the advantages of a bipedal progression. The testis in the adult is made up of a great mass of convoluted tubules arranged in about 200 lobules, and the whole mass is included in a tough fibrous tunic. The lining epithelium of these seminal tubes by a special method of cell division gives rise to the male elements or spermatozoa, which are minute thread-like bodies consisting of a small oval head, about -g-q^^-th inch long, a slender middle-piece nearly as long, and a hair-like flagellum or tail tapering to a fine point and measuring s-^-g-th inch. The tubular system whereby these and the fluid in which they float are conveyed away is derived from the Wolffian tubules, which open into the Wolffian duct, now called vas deferens. Owing to the new position assumed by the testis, this duct has to ascend from the scrotum, to pass through the inguinal canal, and then to descend into the pelvic cavity in order to gain the first part of the urethra, into which it opens. Close to the urethral extremity of each vas deferens, and on its outer side is an irregularly pyriform pouch, the vesicula seminalis, to store the secretion from the testis. In early foetal life the terminal openings of all th< excretory tubes—intestinal, urinary, and genital—uniti and reach the surface by a single slit-like orifice. Thi: condition, which is persistent in the monotremata am Differea- lower vertebrates, is transitory in man am tiatioa Of the higher mammals. When first formed th< ZlZriL h“cler. en<l of the gut, with its appendec ° allantois, has no external aperture, but at ar early date m the embryonic history a pit of the surface integument, which is named the proctodceum, dips in or the ventral side of the vertebral column, and its botton layer comes in contact with the gut close to its extreme blind end. boon the membrane separating this pit froir the gut ruptures, and the cloaca is formed. During the second month of foetal life a transverse ridge, formed by a
downgrowth of the ventral wall of the lower end of the gut, shuts off the allantois from the intestine and divides the cloaca into a ventral or urogenital region, and a dorsal or anal. By the third month this partition has thickened into a definite band of integument which is called the perineum. The urogenital sinus is a slit-like space into which open the allantois, the Wolffian and Mullerian ducts and the ureters. At the ventral end of its aperture an eminence, the tuberculum genitale, rises, deeply grooved on its dorsal surface; and from the lower edge of each lip of the groove a marginal vestibular ridge descends on each side of the opening of the sinus to unite with its fellow at its dorsal extremity. On each side, external to this vestibular ridge, the skin at the margin of the urogenital sinus forms a prominent lip, making the opening appear as a longitudinal fissure. So far the process of development is identical in the two sexes, but at this point differentiafion occurs. In the female the tuberculum genitale becomes the clitoris, and the vestibular and marginal folds respectively the labia majora and minora. In the male the vestibular folds unite in the median line, except at their ventral extremity, thus closing in the canal which forms the second and third parts of the urethra, and in the combined under-wall thus constituted erectile tissue develops, forming the corpus spongiosum urethrae above described. The terminal ventral opening becomes the meatus urinarius. The two cutaneous outer lips also fuse together medially, making the scrotum. The bilateral tuberculum genitale becomes by the development of erectile tissue the corpora cavernosa penis. In each sex the tubular system which does not become functional leaves vestiges which discharge no function. In the female the Wolffian tubules remain as a series of linear streaks in the substance of the broad ligament, the epoophoron. In the male the Mullerian ducts leave two vestiges, a minute saccule appended to the top of the testis, and a minute pouch, the sinus pocularis, in the colliculus seminalis of the urethra. This latter corresponds to the cavity of the uterus in the female, a correspondence which is rendered more striking as this pouch and the adjoining portion of the urethra are surrounded by a mass of unstriped muscle, with some gland tubes included in it, to which the name prostate is given. This organ is about the size and shape of a chestnut. Variability of the Human Body. Is o two human bodies are alike in their organization, and there is no organ in the body which is not liable to vary. Some parts, however, are especially liable to abnormalities, more especially those which have the most complicated developmental history, those which derive their origin from more than one component, which pass through several stages and are late in arriving at their full development. Of those organs which occur in series, the terminal members are more variable than the intermediate. Organs do not vary indifferently in every direction; there are some conceivable forms of variety which never occur others which are common. Nor can variations be reduced to a continuous series. Some of them are sudden breaks, between which and the normal no intermediate links are known. Most varieties are individualistic, depending on conditions of nutrition or environing circumstances affecting development. A few, such as the development of a third incisor or third premolar, seem to be atavistic, but atavism has been invoked in many cases to which it does not apply. The vascular system is the most, and the nervous the least" variable. The muscular and skeletal systems occupy intermediate positions. The literature of anomalies is very large. The general subject is dealt with in the Robert
