402 ROBERT W. HEGNER 



figured (fig. 7, A) was fixed four hours after deposition. The 

 polar bodies have already been produced and the male and female 

 nuclei are in the act of conjugation. The egg consists of a large 

 central mass of yolk and a comparatively thin peripheral layer 

 of cytoplasm, the 'Keimhautblastem' of Weismann. The inter- 

 deutoplasmic spaces are filled with cytoplasm which is connected 

 with the 'Keimhautblastem' by delicate strands of the same 

 material. The enormous amount of yolk' contained in these 

 eggs makes the identification of other substances extremely 

 difficult. The yolk-globules range in size from large deutoplas- 

 mic spheres to small granules, and, as the dissolution of some of 

 them is continually taking place, one is unable to determine 

 where yolk ends and cytoplasm begins. The only accumulations 

 of cytoplasm large enough for examination are those surrounding 

 the nuclei within the yolk mass, and the peripheral layer, the 

 'Keimhautblastem.' No differences in composition or staining 

 qualities were observed between the cytoplasm of these two 

 regions. The 'Keimhautblastem' consists of a fluid ground 

 substance in which are suspended very fine granules. It is a 

 homogeneous layer of cytoplasm everywhere except at the pos- 

 terior end of the egg. At this point there is a disc-shaped mass 

 of larger granules imbedded within the inner portion of it. These 

 granules stain deeply with haematoxylin. They are easily seen, 

 not only in sections but also in eggs that have been properly 

 stained in toto. Because of their ultimate fate I have called 

 these granules the germ-cell determinants (fig. 7, A, g.c.d.). 



The first cleavage divisions take place where the pronuclei 

 fuse. The daughter nuclei move away from each other and as 

 cleavage progresses a separation of the nuclei into two sections 

 occurs. The nuclei of one group form a more or less regular 

 layer equidistant from the periphery; these preblastodermic 

 nuclei (fig. 7, B, jM.n) move outward and fuse with the Keim- 

 hautblastem. Cell walls now appear for the first time and a 

 blastoderm is formed of a single layer of regularly arranged 

 cells. 



The genesis of the pole-cell is as follows: (1) four nuclei ly- 

 ing near the posterior end of the egg are recognized by their 



