382 EMBRYOLOGY. 



closely to them (Cat, Guinea-pig, Mouse, etc.), and take a very 

 prominent part in the composition of the medullary substance. 



There are two antagonistic views relative to the significance of the 

 sexual cords of the primitive kidney, or the medullary cords, in the 

 formation of ova. According to KOLLJKER and ROUGET the medullary 

 cords early fuse with the tubes of PFLUGER and furnish to them the 

 cells which become the follicular epithelium. The cells contained in a 

 follicle would, according to this, come from two sources the follicular 

 cells would arise from the primitive kidney, the eggs from the ger- 

 minal epithelium. Most embryologists dispute this. According to 

 their observations the medullary cords only exceptionally extend close 

 up to a follicle, in many Mammals they do not reach it at all ; 

 consequently not only the primitive ova but also the accompanying 

 follicular cells must be furnished by the germinal epithelium. I also 

 favor the latter view, which appears to me to be best supported by 

 the facts. But what significance the medullary cords have will be 

 better understood when we have become acquainted with the develop 

 ment of the testis, to which we shall now proceed. 



(g) The Testis. 



I will at once state that our knowledge of the development of the 

 testis is less complete than that of the development of the ovary. 



The conditions appear to me to be the clearest in the non-amniotic 

 Vertebrata. We possess here the pioneer researches of SEMPER and 

 BALFOUR on the Selachians, and of HOFFMANN on Amphibia. All 

 these investigators have, with one accord, come to the conclusion 

 that the male sexual products, as well as the female, arise from the 

 germinal epithelium of the body-cavity. In males also there is to 

 be recognised in the region of the primitive kidney a special thickened 

 band of tall epithelial cells, in which are imbedded larger cells with 

 vesicular nuclei, the primitive spermatic cells. In the Sharks, the 

 conditions of which I shall make the basis of the further description, 

 they form irregular cords of cells, the " Yorkeimketten " of SEMPER 

 (fig. 218 A). Out of these are developed small, spherical, follicular- 

 like bodies (fig. 218 B), by the ingrowth of surrounding connective 

 tissue into the cords, which are thereby divided up. 



Thus far, therefore, complete agreement exists in the development 

 of both kinds of sexual products. But whereas in the case of the 

 ovary one cell in each follicle increases in size and is converted into 

 the ovum, a like process does not take place in the male ; here the 



