Germinal Layers in Vertebrates. 369 



nuclei of the lower blastoderm cells, were termed merocytes 

 or periblast nuclei. The presence of so many nuclei without 

 the outlines of cells is probably to be exj)lained as being due 

 to rapid nuclear division without corresponding cell-division. 

 I must lay stress on the fact that these nuclei do not perish, 

 as some investigators would have us believe, but form new 

 cells, which take part in the building-u|) of the embryo. I 

 shall designate these nuclei simply as yolk-nuclei. 



After the formation of the blastoderm a more or less distinct 

 boundary may be noticed between the blastoderm and the 

 yolk, or, more strictly speaking, between the blastoderm and 

 the intermediate layer wliich belongs to the yolk. Yet we 

 may observe all the time the equatorial division of the yolk- 

 nuclei and the formation of new cells, which become con- 

 stricted off from the yolk and join the blastoderm cells. Now, 

 after the overgrowth of tlie yolk by the blastoderm cells has 

 commenced it is time to distinguish the primary germinal 

 layers. I consider the entire 'blastoderm, the cells of which 

 grow over the yolk, as the ectoderm, but the yolk with the 

 yolk-nuclei must be termed the endoderm, since the intestine 

 arises from these vitelline elements. 



In passing on to the description of the further develop- 

 mental processes which usher in the formation of the noto- 

 chord and the mesoderm it is necessary for me to draw 

 attention to the fact that in none of the Teleosteans which I 

 have examined have I observed an invagination. It is well 

 known that certain investigators maintain that the mesoderm 

 in Teleostean fishes is formed by invagination, while others, 

 on the contrary, assert that it arises by cleavage of the blasto- 

 derm cells. Although I myself have not observed any 

 invagination, I think I may assume that no essential differ- 

 ence exists between the two processes as they are described 

 by different investigators; for the invagination in this case 

 also has no connexion with the formation of the intestine, 

 and is nothing else than the forward growth of the reflected 

 edge of the blastoderm. In both cases thei'efore the same 

 blastodermal or ectodermal elements are engaged, and it is 

 a matter of secondary importance whether the cells are from 

 the outset arranged in such a way that the continuous rudi- 

 ment of the notochord and mesoderm arises by splitting off 

 from the upper portion of the blastoderm (the rudiment of the 

 nervous sj'stem), or whether this rudiment is formed by the 

 forward displacement of the cells starting from the margin of 

 the enveloping layer. Whether by means of the process of 

 splitting-off or by that of so-called invagination, the con- 

 tinuous rudiment of the notochord and mesoderm is equally 



