Germinal Layers in Vertebrates. 371 



the jolk-nuclei. On the other hand, while in this manner an 

 important role is ascribed to the yolk-nuclei by some investi- 

 gators, the others deny that they take any share in the 

 formation of the embryo. From my own results 1 am led to 

 suppose that not only the intestine, but also the entire endo- 

 derm, arises from the yolk-nuclei, while the blastoderm cells 

 represent the ectoderm. The formation of the intestine from 

 the yolk-nuclei is so distinct in all stages that I cannot under- 

 stand how this fact can be disputed. But, on the other hand, 

 I must corroborate the assertion that a kind of invagination 

 (blastoderm al fold) is to be observed at the posterior margin 

 of the germinal disk. My results consequently reconcile to a 

 certain extent the statements of different investigators, for I 

 confirm both the origin of the intestine from the yolk-nuclei, 

 as also the presence of the invagination. But in the present 

 instance also this invagination or doubling-down of the blasto- 

 derm cells has nothing to do with the formation of the 

 intestine. I therefore see in it no process of gastrulation, but, 

 as in the case of other Vertebrates, the ingrowth of the ecto- 

 blastogenous rudiment of the notochord and mesoderm, which 

 proceeds in a forward direction from the posterior margin of 

 the enveloping layer. The Selachians agree with the Tele- 

 ostean fishes also in the fact that not only is the inversion of 

 the blastoderm cells to be observed at the posterior margin of 

 the enveloping layer, but also the splitting-off of cells here 

 and there. These two different processes — the formation of 

 the intestine from the yolk-nuclei and the ectoblastogenous 

 invagination — are accomplished simultaneously in such a way 

 that, while the ectoderm cells are invaginated at the posterior 

 margin of the enveloping layer, the continuous sheet of endo- 

 derm is formed from the yolk-cells, which give rise to the 

 intestine. The invaginating blastoderm cells in no way con- 

 stitute the intestinal wall ; they merely lie, while they grow 

 inwards, so closely upon the true endoderm cells which have 

 arisen from the yolk-nuclei, that the two different rudiments 

 — the invaginated ectodermal one and the endodermal which 

 has developed on the spot from the yolk-nuclei — are intimately 

 connected one with another. It is the more difficult to draw 

 a dividing line between the two rudiments, since in the lateral 

 rudiments of the mesoderm both kinds of cells are in close 

 connexion. 



The rudiment of the notochord is excluded from the limita- 

 tion of the intestinal cavity by the actual endoderm cells, and 

 forms for a time an appendage of the dorsal wall of the intes- 

 tine. As regards the mesoderm, we may follow Riickert in 

 distinguishing the axial and peripheral portions of this layer. 



