EXPERIMENTS WITH FROGS EGGS. 



77 



this time the black hemisphere of the egg was much flattened, 

 and sections show, Fig. 13, that the fluid of the blastocuel has 

 been completely driven out of the egg. The figure shows that 

 not only has the top of the egg been flattened, but the small cells 

 at the sides of the blastoccel have moved over across the top of 

 the yolk, so that a thick layer of small cells lies as a flat plate on 

 top of the larger yolk cells. Here and there a small crack or space 

 is present along the region of contact of the large and the 

 small cells. Not infrequently the top of the egg is thrown over 

 as a fold, as shown in Fig. 14. This is due no doubt to the fact 

 that as an arch it occupies more space than when flattened down 

 on the egg. In the preserved eggs an artefact often appears 

 beneath the outer layer of cells. 



FIGS. 13-17. Eggs of Kana sylrestris after having been rotated at 1, 600 revolu- 

 tions per minute. 13, immediately after rotating ; 14, top of another egg at same 

 time ; 15-17, later stages when gastrulation has begun. 



