DEVELOPMENT OF THE URINARY AND GENERATIVE ORGANS 1209 



organ; it soon loses its connection with the central mass, and a tunica albuginea 

 develops between them. The ova are chiefly derived from the cells of the central 

 mass; these are separated from one 

 another by the growth of connective 

 tissue in an irregular manner; each ovum 

 assumes a covering of connective tissue 

 (follicle) cells, and in this way the rudi- 

 ments of the ovarian follicles are formed 

 (Fig. 1113). According to Beard the 

 primitive ova are early set apart during 

 the segmentation of the ovum and migrate 

 into the germinal ridge. 



Wolffian body 

 Mullerian duct 



Woljfian duct 



Waldeyer taught that the primitive germ 

 cells are derived from the "germinal epithe- 

 lium," covering the genital ridge. Beard, 1 on 

 the other hand, maintains that in the skate they 

 are not derived from this epithelium, but are 

 probably formed during the later stages of cell 

 cleavage, before there is any trace of an embryo; 

 and a similar view was advanced by Nussbaum 

 as to their origin in amphibia. Beard says: "At 

 the close of segmentation many of the future 

 germ cells lie in the segmentation cavity just 

 beneath the site of the future embryo, and there 

 is no doubt they subsequently wander into it." 

 The germ cells, "after they enter the resting 



phase, are sharply marked off from the cells of the embryo by entire absence of mitoses among 

 them." They can be further recognized by their irregular form and ameboid processes, and by 



Medulla spinalis 



Spinal ganglion 



Notochord 



Sympathetic ganglion 

 Inferior vena cava 

 Common iliac artery 

 Ureter 

 Mesovarium 



Intestine 



Bladder 

 Umbilical artery 



FIG. 1111. Transverse section of human embryo eight 

 and a half to nine weeks old. (From model by Keibel.) 



Uterine tube 



Germinal epithelium 



Medulla 



Rcte 



Epoophoron 



c ^ 

 3^"" 



r ot 



Mesonephros 



Plica peritonalis 

 tubce 



Uterine tube 



Fio. 1112. Longitudinal section of ovary of cat embryo of 9.4 cm. long. Schematic. (After Ccert.) 

 'Journal of Anatomy and Phys ology, vol. xxxviii. 



