EXPERIMENTS WITH FROGS EGGS. 65 



egg has concentrated in a cap. It represents all of the ectoderm 

 of the embryo. This brings the development to the stage at 

 which my former series begins. If, after this stage is reached, 

 the ectodermal material at the top of the egg turns into the inte- 

 rior, instead of growing down over the surface, as in the normal 

 egg, an invaginated embryo is formed. 



FIGS. 26-31. Segmentation stages of egg in lithium chloride. A'ana palustris. 



If it fails to turn in, it remains as a solid cap at the top, and if 

 not too condensed the anterior end of the neural plate develops. 



In the lithium types there is rather a sharp line of demarcation 

 between ectoderm, mesoderm and endoderm. The three layers 



