446 CHARLES H. SWIFT 



mitochondria. He believed that the primitive cells or blasto- 

 meres possessed this type of mitochondria and that as the tissues 

 were differentiated from them, the cells of the tissues acquired 

 the rod-shaped mitochondria. In this way he was able to dis- 

 tinguish the germ-cell at any stage in the 3^oung embryo and 

 differentiate it from the ordinary somatic cell. By this means 

 he came to the conclusion that the definitive sex-cells are de- 

 scended from the primordial germ-cells. 



Firket in 1914 published a very complete account of the or- 

 ganogenesis of the indifferent gonad and ovary in the chick, which 

 was the last article dealing with the primordial germ-cells and 

 their relation to the definitive ova. According to Firket ('14) 

 a great majority of the definitive ova were derived from the cells 

 of the germinal epithelium, but he admitted that there was no 

 reason why certain of the primordial germ-cells might not 

 produce some of the definitive ova. Firket distinguished two 

 lines of germ-cells — the so-called primary gonocytes or primordial 

 germ-cells and the secondary gonocytes which are derivatives of 

 the germinal epithelium. As the chick embryo advanced in 

 development a majority of the primary gonocytes degenerated 

 and those which remained, having lost their vitellus, and other 

 distinguishing characters, could not be separated from the 

 secondary gonocytes. 



Firket thought it necessary to consider the primary gonocytes 

 of Vertebrates "comme etant un rappel phylogenique des gono- 

 cytes definitifs des classes inferieurs notamment des Cyclostomes 

 et des Acraniens." 



Any further account of the earlier literature on the subject 

 of the origin of the definitive ova would be useless. The reader 

 is referred to the excellent bibliographies of Bouin ('00) and 

 Firket ('14) for the articles on this subject. 



MATERIAL AND METHODS 



The chick was selected as the animal best adapted to this 

 investigation for several reasons: 1) Because of the ease with 

 which material may be procured. 2) As will be seen from 

 the preceding paragraphs the question as to the origin of the 

 definitive ova in this species is still an open one. Especially 



