406 ROBERT W. HEGNER 



plainly evident. Sixteen pole-cells were present at this time. 

 After a mitotic division, during which the pole-disc granules 

 are apparently approximately equally divided between the 

 daughter cells (fig. 8, B), the pole cells are smaller, but, although 

 no larger than the neighboring blastoderm cells, they may still 

 be distinguished by the presence of the pole-disc granules and 

 also by the larger nucleus containing a lesser number of chro- 

 matin granules. Pole-disc granules are still faintly visible at a 

 later period when the germ cells are migrating into the embryo 

 through the pole-cell canal, and still later, as figure 8, E shows, 

 the germ cells can be distinguished easily from the ectoderm and 

 mesoderm cells although the pole-disc granules have entirely 

 disappeared. 



The pole-disc. The pole-disc varies somewhat in compactness 

 but in most cases appears in section as shown in figure 7, D. 

 Nothing resembling it occurs in other parts of the egg. Its 

 granules are very susceptible to stains and can be made visible 

 by means of a number of different dyes. The 'Keimwulst' of 

 Chironomus (Ritter '90), the 'Dotterplatte' of Calliphora (Noack, 

 '01; fig. 2) and the 'Keimbahnplasma' of Chironomus (Hasper 

 '11; fig. 1) all present a similar appearance. In these forms, as 

 well as in the Chrysomelid beetles I have studied, all or nearly 

 all of the granules (fig. 7, C) are taken out of the egg by the pole- 

 cells. Wieman ('10a) however, gives a figure showing a section 

 of the posterior end of the egg of Leptinotarsa signaticollis after 

 the protrusion of the pole cells, in which there is still represented 

 what he calls the pole-disc. The fact that the mass of granules 

 described by Wieman does not resemble the pole-disc as I have 

 found it, nor other similar accumulations in insect eggs (Keim- 

 wulst, Dotterplatte, Keimbahnplasma) and the statement that 

 "the grarmles are not all taken up by the cells in their migration 

 and the greater part of them remains behind after the cells have 

 passed through" (Wieman, '10a, p. 186), a condition contrary 

 to that described by every one of the writers cited above, lead 

 to the conclusion that Wieman has confused something else for 

 the pole-disc. This seems all the more probable, since the species 

 studied by Wieman, namely Leptinotarsa signaticollis, is very 



