Oerminal Layers in Vertebrates. 365 



My investigations upon the formation of the germinal layers 

 in Petromyzon commence with the stage of the blastula. The 

 blastula includes an excentric cavity, lying nearer the upper 

 pole, the roof of which is constituted by smaller blastoderm 

 cells and the floor by several layers of larger cells, richer in 

 yolk, which develop into endoderm. I was also able to 

 observe the multiplication and extension of the blastoderm 

 (ectoderm) cells over those richer in yolk (endoderm cells). 

 Numerous mitoses may be observed in the ectoderm cells. I 

 counted the mitoses in a series of sections through a stage of 

 this kind, and found that the endoderm cells contained five 

 and the superficial ectoderm cells twenty-three mitoses. In 

 this manner smaller ectodermal cells surround the larger 

 endodermal elements, which thus come to lie within and 

 become enveloped by the former. Simultaneously with this 

 on one side, which becomes the dorsal surface of the embryo, 

 a particularly active multiplication of the ectoderm cells may 

 be noticed, and here, at a spot which marks the posterior end 

 of the embryo, the invagination of the ectoderm cells com- 

 mences. By this means a cavity is formed, which is usually 

 termed the cavity of the gastrula, or archenteron, and the 

 dorsal wall of which is constituted by the invaginated ecto- 

 derm cells. But this invagination forms only the dorsal wall 

 of the cavity, and I therefore term it the dorsal invagination. 

 The ventral wall of the cavity is formed by the endoderm 

 cells, which are not invaginated, but were previously situated 

 here. These conditions can be seen with great distinctness 

 in median sections through stages of this kind. In such 

 sections we observe that the ectoderm cells grow inwards from 

 the dorsal margin of the enveloping laj'er, and form the dorsal 

 wall of the cavity ; the ventral wail, on the other hand, is 

 formed by the cells of the endoderm. We also observe the 

 different character of the cells of the dorsal and ventral v,^alls. 

 The former are arranged in epithelial fashion, and form the 

 continuation of the ectoderm cells, which grow inwards from 

 the margin of the enveloping layer. The cells of the ventral 

 wall of the cavity have a roundish or polyhedral form, and 

 are not arranged in epithelial fashion. They acquire the 

 regular epithelium-like character later on, when the intestine 

 arises from them. In the enumeration of the mitoses in a 

 series of sagittal sections it was found that the endoderm cells 

 contained four (chiefly in the anterior portion of the cavity), 

 the ectoderm cells twenty-four (chiefly upon the dorsal side ; 

 among these were seven at the margin of the enveloping layer) , 

 and the invaginated ectoderm cells four mitoses. 



I must lay quite special emphasis upon the fact that the 



