ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 243 



The strands of the second proliferation — the cortical strands — enter 

 into the composition of the cortical zone of the ovary. It is from their 

 elements that the definitive ova are formed. These are large, as in all 

 animals with meroblastic ova [but some Teleostean ova cannot be called 

 large ?] ; they are directly surrounded by follicular epithelium ; there is 

 no follicular cavity. The first cortical zone corresponds to the primary 

 cortical zone in Mammals. There is not in birds any third proliferation 

 of the germinative epithelium. A few cortical strands degenerate, and 

 this seems to point towards the complete retrogression of these structures 

 in Mammals, such as the cat. In exceptional cases the cortical zone 

 does not completely surround the medullary zone, which may be com- 

 pared with what is normal in the ovary of the mole. 



There is a duality of origin in the gonocytes of birds. The primary 

 gonocytes appear very early in the course of ontogeny, before the for- 

 mation of the definitive genital primordium. The secondary gonocytes 

 differentiate at the expense of cells of the ccelomic epithelium, which, 

 because of this, deserves the name of germinative epithelium. The 

 primary gonocytes are from the first independent of the cells which 

 surround them. This independence is marked by the faculty they have 

 of changing their position within the tissue by amoeboid movements, 

 and by the absolute parallelism of development in the case of gonocytes 

 occupying quite different positions. The primary gonocytes pass through 

 the typical phases of other sex-cells. They have the value of oogonia 

 till the eleventh day of incubation in the f enfale embryo. Most of them 

 degenerate. It is impossible to determine whether in the cortical zone 

 of the ovary some of the primary gonocytes may not attain to maturity. 

 The fact is that the primary gonocytes have mainly a phyletic in- 

 terest. They are the vestiges of elements which became definitive ova 

 in Cyclostomes and Acrania. The history in Amphibians is much 

 the same as in Birds. The granular form of chondriosomes, which 

 Rubaschkin regards as characteristic of the gonocytes, is not a constant 

 character. 



In the development of the ovary, the first step, occurring at the end 

 of the second day, is the appearance of the primary gonocytes. They 

 appear on both sides of the middle line in the splanchnopleure of the 

 24-30th somites. They approach the middle line, reach the internal 

 side of the Wolffian body, and penetrate into a differentiated portion of 

 the coelomic epithelium formerly called the genital primordium. 



Subsequently there are formed the " urogenital connexions," that is to 

 say the rudiment of the rete ovarii. They differentiate within the 

 mesenchyme from a special portion of mesoblast, as already noted. 



The rete having been formed, all that remains is due to the germi- 

 native epithelium— a specialized portion of the peritoneal epithelium 

 which gives origin by successive proliferations to cellular buds. The 

 first formed are the future medullary strands ; these are followed by 

 epithelial strands of the second proliferation. Some elements of the 

 ovary show retrogressive changes, others progressive changes. The 

 medullary strands illustrate the former, the cortical strands the latter. 

 There is a struggle among the possible ova, and only a small percentage 

 attain to maturity. 



