No. 3.] SEGREGATION OF THE SEX-CELLS. 483 



consists of a few (less than twenty) large nuclei lying in the 

 cortical layer of the yolk. The remaining development is em- 

 phasized chiefly by the formation of the very large intestine, 

 the early communication of the intestine with the exterior by 

 means of the first gill-slit, which is open almost as soon as the 

 larva is hatched, and the enormous Kupffer's vesicle which 

 sometimes reaches a length equal to one-half that of the yolk. 

 The early connection of Kupffer's vesicle with the forming 

 intestine and the presence (occasional ?) of a neurenteric canal 

 connecting Kupffer's vesicle with the neural canal leaves no 

 doubt of the homology of this structure with the post-anal vesi- 

 cle of Elasmobranchs. 



The sex-cells can first be distinguished from the surrounding 

 cells about the time the blastopore closes. The earliest ones 

 distinguishable, exclusive of abnormal cases, are from an ovary 

 in the eggs of which the blastopore is not yet closed, or just 

 closed (Fig. 1), and in which the mesoderm is not yet split off 

 from the entoderm. Only two cells which can with certainty 

 be said to be sex-cells are seen in one of these eggs (Fig. 1). 

 They differ from the surrounding cells in having well-defined, 

 rounded outlines, and in the distribution of the chromatin in 

 the nucleus. The chromatin of the surrounding cells is col- 

 lected in one, or, if the cells are undergoing division, in two or 

 three masses. The chromatin of the sex-cells is uniformly 

 distributed in small granules. The protoplasm of the smaller 

 egg stains somewhat deeper than that of the surrounding 

 cells. The larger egg has a large, light court surrounding 

 the nucleus ; the peripheral substance is stained, but slightly dif- 

 ferent from that of surrounding cells. The protoplasm of both 

 eggs contains yolk granules similar to those found in the blas- 

 todisc. These eggs measure as follow : nucleus 5 fi ; diameter 

 of smaller egg, 13 /x; diameter of larger egg, 15 /j, x 18 fj,; its 

 court, 13 p. 



In another egg of this ovary, in which the blastopore is closed, 

 a sex-cell similar to the smaller one described lies just at the 

 margin of the future mesodermic cells. 



In a third egg there are two large cells lying on the yolk. 

 Not all eggs of this stage show the sex-cells equally well. In 

 some eggs in which the blastopore is closing none can, with 

 certainty, be made out. 



