342 



LEWIS R. GARY. 



materials. The plasma-like substance has disappeared for a con- 

 siderable proportion and the yolk spheres have undergone an 

 apparent disintegration. Their outlines are no longer distinct, 

 as in the earlier stages, and in many instances, they can be ob- 





FIG. 4. Blastula of A. bermudensis just at the beginning of the invagination. 



served breaking up into rather coarse granules which are being 

 spread out through the surrounding plasma-like material. 



In the stage shown in Fig. 5 the process of invagination has 

 gone on until the section shows a w'ell-markecl early gastrula, 

 and puts beyond any question the type of endoderm formation 

 in this species. The character of the cells in the invaginating 



^^' V-;-" ' Ipp^ 



I' ii.. 5. Invagination of A. bermudensis. 



area has not as yet undergone any change to distinguish them 

 structurally from those making up the ectoderm. 



The nutritive material within tlu- original blastoccel shows 

 practically the same conditions as nok-d for the stage last de- 



