THE URINARY AND GENERATIVE ORGANS. 137 



lary part of the organ is of epiblastic origin, and is derived from the tissues 

 forming the sympathetic ganglia of the abdomen, while the cortical portion is of 

 mesoblastic origin, and originates in the mesoblast just above the kidneys. The 

 two parts are at first quite distinct, but become combined in the process of devel- 

 opment. The suprarenal capsules are at first larger than the kidney, but become 

 equal in size about the tenth week, and from that time decrease relatively to the 

 kidney, though they remain, throughout foetal life, much larger in proportion 

 than in the adult. 



Development of the Generative Organs. The first appearance of the repro- 

 ductive organs is essentially the same in the two sexes, and consists in a 

 thickening at one spot of the epithelial layer which lines the peritoneal or 

 body cavity, with a slight increase of the connective tissue beneath it, form- 

 ing a low ridge. This is termed the genital ridge, and is situated on the 

 mesial side of each Wolffiaa body, and from it the testicle in the one sex, and 

 the ovary in the other, are developed. The ridge, as the embryo grows, grad- 

 ually becomes pinched off from the Wolffian body, with which it is at first 

 continuous, though it still remains connected to the remnant of this body by a 

 fold of peritoneum, the mesorchium or mesocuriuni. About the seventh week the 

 distinction of sex begins to be perceptible. The epithelium on the genital ridge, 

 which is called " germ-epithelium," in the female becomes distinctly columnar, 

 multiplies rapidly, and begins to form primitive ova, in a manner presently to be 

 described ; whereas in the male, though the germ-epithelium has a tendency to 

 become columnar, the cells are, on the whole, flatter and smaller than in the 

 female. 



Development of Male Organs. The tubuli seminiferi of the testicle appear at 

 an early period. It is believed that they are formed by the extension of epithelial 

 cells on the surface of the genital ridge into the connective tissue or stroma on 

 which they rest; rows of cells are thus developed which become the lining cells 

 of the seminal ducts. From the mesonephros tubules grow toward the kidney, 

 filtering into relation with the seminal ducts and forming the tubuli recti and 

 rete testis, through which the semen escapes from the testis and passes into the 

 tubules of the upper part of the mesonephros, which persist as the epididymis, 

 and thence make their way to the urethra (urogenital sinus) by the Wolffian 

 duct, which becomes the vas deferens and ejaculatory duct of the adult. 



The Miillerian ducts disappear in the male sex, with the exception of their 

 lower ends. These unite in the middle line, and open by a common orifice into 

 the urogenital sinus. This constitutes the uterus mascuUnua or sinus prostaticus. 

 Occasionally, however, the upper end of the duct of Miiller remains visible in 

 the male, constituting the little pedunculated body, called the hydatid of the 

 epididymis, sometimes found in the neighborhood of the epididymis, 1 between 

 the testes and globus major. 



It has been seen that the upper portion of the mesonephros and the Wolffian 

 ducts persist. The rest of the mesonephros disappears almost entirely, a few of its 

 tubules forming the vas aberrans and a structure described by Giraldes, and 

 called, after him, " the organ of Giraldes," which bears a good deal of resem- 

 blance to the organ of Rosenmiiller in the other sex. It consists of a number of 

 convoluted tubules lying in the cellular tissue in front of the cord, and close to 

 the head of the epididymis. 



The descent of the testis and the formation of the gubernaculum are described 

 in the body of the work. 



Development of Female Organs. The ovary, as above stated, is formed from 

 the genital ridge, which becomes pinched off from the remains of the Wolffian 

 body, but is still attached by a mesovarium. It consists of a central part of con- 

 nective tissue covered by a layer of germ-epithelium, from which the ova are 

 developed. This epithelium undergoes repeated division, so that it rapidly increases 



1 Mr. Osborn, in the St. Thomas's Hospital Reports, 1875, has written an interesting paper point- 

 ing out the probable connection between this foetal structure and one form of hydrocele. 



