SEX-DIFFERENTIATION IN XIPHOPHORUS HELLERI. 73 



origin of the definitive germ cells, he says : " In the interval be- 

 tween the first and second larval sexual cycle following the degen- 

 eration of large numbers of maturation cells the gonad becomes 

 filled with small cells which, because of their size, nuclear struc- 

 ture, and staining capacity, appear as transition stages between 

 mesothelial cells (germinal epithelium and sex-cord elements) and 

 true germ cells. The later history shows them to be germ cells, 

 but their origin is open to two interpretations and is not as clear 

 as could be desired. The writer considers these cells as small germ 

 cells descendants of the primordial sexual elements, and not as 

 transformed germinal epithelium elements, but admits that the 

 evidence from his material is equally strong for the germinal 

 epithelium viewpoint." 



Clearer cut results have been obtained by Firket (1920) in the 

 albino rat. The primordial germ cells disintegrate and have disap- 

 peared entirely in the testis of the albino rat from the tenth to the 

 fifteenth day after birth. The origin of the definitive germ cells 

 in the albino rats, according to Firket, are as follows : " At the 

 time the first spermatogonia appear they are easily recognizable by 

 the texture of their nuclei and are very numerous. This has been 

 shown very distinctly in the same species by Hoven. Let us insist 

 that those spermatogonia can only be derived from the small epi- 

 thelial cells, as they are at this stage the only type of cells present 

 in sufficient numbers in the sex-cord. The spermatogonia must 

 be called ' secondary germ cells.' ' 



It was stated previously (p. 47) that the data presented in this 

 paper comprise only a part of the study of the sex problem of 

 Xiphophorus hellerl. Particular interest is attached to the experi- 

 mental work in progress at the present time. Closer cytological 

 study of the material is imperative to elucidate difficulties encoun- 

 tered in cell division, chromosome composition, etc. A discussion 

 of the bearing of the present data on the theory of sex will be post- 

 poned until the completion of the entire problem. 



XI. SUMMARY AND CONCLUSIONS. 



1. At birth the young fish measure on the average 8 mm. in total 

 length and are on the verge of sex-differentiation. 



2. The gonads of the indifferent stage are paired and widely 



