484 EIGENMAN/V. [Vol. V. 



In another ovary, in an egg in which the blastopore is not 

 yet closed, large cells are seen in the entoderm and also in the 

 ectoderm (Fig. 2). In fact, if the large cells were not so numer- 

 ous in the ectoderm, the conclusion would naturally suggest 

 itself that the sex-cells really arise in the ectoderm and migrate 

 to the entoderm before mesoderm is formed at all. There are, 

 however, too many large cells in the stage shortly preceding 

 the closing of the blastopore in the outer portion of the blasto- 

 derm. The fact is, the inner cells of the blastoderm segment 

 more rapidly toward the close of segmentation than the outer 

 cells, and many of the outer cells become distended through 

 intracellular digestion of the surrounding ovarian fluid. The 

 outer cells, therefore, and, when the two-layered condition is 

 reached, the cells of the ectoderm, are larger than those of the 

 entoderm. It is still possible, however, that the sex-cells arise 

 in the ectoderm. If, however, they are segregated as early as 

 the fifth segmentation, that is, before there is any differentiation 

 into ectoderm and entoderm, as is probable, it seems to be of 

 no great importance whether at the time of the separation of 

 the blastoderm into ecto- and entoderm the cells lie in its outer 

 or inner portion. 



The eggs of the last-mentioned ovary were killed in the strong 

 solution of osmic, chromic, acetic acid, and stained with Gren- 

 adier's alcoholic borax carmine. This method of staining does 

 not differentiate the sex-cells. They can only be distinguished 

 by their size. They measure 18 fi. 



In the eggs of another ovary, in which the cells have become 

 heaped up in the head region, the chorda and neural thicken- 

 ings are formed for some distance back, and the mesoblast is 

 separated from the entoderm by a well-marked line ; the sex-cells 

 stand out from the surrounding cells with great prominence 

 (Figs. 3 and 4). This is not due to any marked change in the 

 sex-cells themselves, but rather to the fact that the surrounding 

 cells have undergone further division and are crowded so that 

 the boundaries are not defined, while those of the rounded sex- 

 cells are well marked. 



The largest and most conspicuous cell of this stage lies in the 

 mesoblast just beside the chorda. It measures 18 X 23 /*, and has 

 a nucleus measuring about 6 fi. On comparing this size with 

 segmenting eggs, it is found that it agrees in size with some of 



