THE URINOGENITAL SYSTEM. 349 



Primitive Ova. The true generative products make their 

 first appearance as the primitive ova between stages I and K. 



In the sections of one of my embryos of this stage they are 

 especially well shewn, and the following description is taken 

 from those displayed in that embryo. 



They are confined to the region which extends posteriorly 

 nearly to the end of the small intestine and anteriorly to the 

 abdominal opening of the segmental duct. 



Their situation in this region is peculiar. There is no trace 

 of a distinct genital ridge, but the ova mainly lie in the dorsal 

 portion of the mesentery, and therefore in a part of the mesoblast 

 which distinctly belongs to the splanchnopleure (PL 12, fig. 140). 

 Some are situated external to the segmental involutions ; and 

 others again, though this is not common, in a part of the 

 mesoblast which distinctly belongs to the body-wall (PL 12, 

 fig. 14^). 



The portion of mesentery, in which the primitive ova are 

 most densely aggregated, corresponds to the future position of 

 the genital ridge, but the other positions occupied by ova are 

 quite outside this. Some ova are in fact situated on the outside 

 of the segmental duct and segmented tubes, and must therefore 

 effect a considerable migration before reaching their final positions 

 in the genital ridge on the inner side of the segmental duct 

 (PL 12, fig. 14$). 



The condition of the tissue in which the ova appear may at 

 once be gathered from an examination of the figures given. 

 It consists of an irregular epithelium of cells partly belonging 

 to the somatopleure and partly to the splanchnopleure, but 

 passing uninterruptedly from one layer to the other. The cells 

 which compose it are irregular in shape, but frequently columnar 

 (PL 12, figs. 140 and 14^). 



They are formed of a nucleus which stains deeply, invested 

 by a very delicate layer of protoplasm. At the junction of somato- 

 pleure and splanchnopleure they are more rounded than else- 

 where. Very few loose connective-tissue cells are present. The 

 cells just described vary from X)o8 Mm. to 'Oi Mm. in diameter. 



The primitive ova are situated amongst them and stand out 

 with extraordinary clearness, to which justice is hardly done in 

 my figures. 



