186 



UROGENITAL SYSTEM 



at a later stage of development, the bladder dilates and the ureters are drawn 

 apart, the upper part of the crista is expanded into the trigone of the bladder. 



DEVELOPMENT OF THE GENITAL GLANDS AND DUCTS 



The genital glands develop comparatively late, on the mesial aspects of the 

 Wolffian bodies (figs. 226, 237). Here the coelomic epithelium becomes thickened 

 to form what is known as the germinal epithelium (Waldeyer). The cells become 



FIG. 234. SECTION THBOUGH A HUMAN EMBRYO OF 30 MM. ABOUT THE BEGINNING OF THE 

 THIRD MONTH. (T. H. Bryce.) 



On each side of the spinal oord are seen the cartilages of the neural arch ; below the cord the body 

 of j the primitive vertebra with the notochord passing through it ; between the two are the spinal 

 ganglia, from which extend downwards the spinal nerves. Below the vertebra the cardinal veins 

 are united by a large anastomosis ; below this the two common iliac arteries. Between the iliac 

 arteries the mesentery leaves the posterior abdominal wall ; on each side of this the ureters. External 

 to the ureters, the Wolffian bodies. On the mesial aspect of each is seen the genital gland connected 

 by its mesentery; on the outer aspect of each, the Wolffian mesentery containing, to the inner side 

 the Wolffian, to the outer side the Miillerian duct. Projecting into the abdominal cavity from 

 below, the allantoic duct attached by a mesentery, in which the two allantoic arteries run. The 

 intestine is cut in several places, and on each side are seen the lateral lobes of the liver. 



columnar and arranged in several layers, while the underlying mesenchyme 

 becomes somewhat thickened along a projecting ridge, the genital ridge. The 

 germinal epithelium by proliferation of its cells increases in depth, and among the 

 proper epithelial elements certain larger and more spherical cells appear (fig. 226, 



