SEX-DIFFERENTIATION IN XIPHOPHORUS HELLERI. 49 



lining of the body cavity. The gonad inside of this sac consists 

 of two kinds of cells : 



1. Primordial germ cells. 



2. Very much smaller cells with elongated nuclei, which can not 

 be distinguished from the peritoneal cells and which tend to sur- 

 round the germ cells to form follicles (Fig. i). 



The primordial germ cells are unmistakable. They are the 

 largest cells in the body of the fish and measure on the average, 

 cell 14.4/1, nucleus 8.4/1,, in diameter. The nucleus takes a lighter 

 stain than the cytoplasma and possesses one more or less distinct 

 nucleolus. The chromatin occurs most commonly in strands or 

 loops immediately beneath the nuclear membrane. The germ cells 

 may be single or in nests of two or more cells. 



As stated above, the pre-natal developmental changes have not 

 been studied systematically. Several stages, however, are on hand. 

 The gonads assume the bilateral position at about 3.2 mm. in total 

 length. Before that the germ cells are in a single mass placed 

 medially in the body cavity. The peritoneum already surrounds 

 the gonad more or less loosely and here the mingling of the peri- 

 toneal cells with the germ cells is very evident (Fig. 3). As these 

 rather small insignificant cells, mingling with the primordial germ 

 cells, play by no means an insignificant role in later differentiation, 

 I have been particularly anxious to trace their origin, and must 

 conclude that no criterion whatever separates them from peritoneal 

 cells. This view is consonant with that of McLeod (1881), Jun- 

 gerson (1889), and Eigenmann (1897). The primordial germ 

 cells of the 3.2 mm. stage are very conspicuous structures. The 

 cell is ii.Sfj. and the nucleus is 6.4/1, in diameter. The nucleus 

 may be tabulated and appear as if more than one nucleus was 

 present in a cell (Fig. 3). The cytoplasm is yolk laden. In short, 

 the conditions of these cells indicate an early segregation. 



IV. EARLY SEX-DIFFERENTIATION OF THE FEMALE. 

 The earliest stage of the ovary shows very little difference from 

 the indifferent gonad. In the early development of the ovary the 

 primordial germ cells grow in size and form follicles. At the time 

 these follicles have reached approximately the medium size they 

 degenerate and are absorbed. This process is referred to as retro- 

 4 



