Germinal Layers in Vertebrates. 367 



cavity. The intestine is formed by the divergence and 

 displacement of the endoderm cells, which are not invaginated, 

 but were previously situated here, and may be distinguished 

 all the time from the smaller ectoderm cells by their relative 

 size, by the amount of the yolk-granules which they contain, 

 and to a certain extent by their want of pigment. While the 

 dorsal invagination is taking place, the cavity which subse- 



3uently becomes that of the intestine is gradually formed by 

 ivergence of the endoderm cells. The formation of the wall 

 of this cavity proceeds in precisely the same manner as in 

 Amphioxus and Petromyzon, i. e. the endoderm cells separate 

 in such a way that they first give rise to the ventral and 

 lateral boundary of the intestine, while the dorsal portion is 

 still open. Thus it comes to pass that the rudiment of the 

 notochord, which has separated from the lateral mesodermal 

 rudiments, temporarily takes part in the limitation of the 

 enteric cavity. Subsequently the endoderm cells unite 

 beneath the notochord, which in this manner is again 

 excluded. As regards the lateral portions of the dorsal plate- 

 like ingrowth, which represent the ectoblastogenous rudiment 

 of the mesoderm, they are from the first excluded from the 

 delimitation of the enteric cavity owing to the intervening 

 endoderm cells, part of which attach themselves to the ecto- 

 blastogenous mesoderm cells in order to furnish their contri- 

 bution to the formation of the mesoderm. 



I must not conceal the fact that these conditions are not 

 exhibited by all Amphibians. According to the statements 

 of certain investigators the Anura are an example of an 

 instance in which the notochord is excluded from the outset 

 from the delimitation of the enteric cavity. Unfortunately I 

 Lave no experience myself in this direction, since I have not 

 studied the development of the Anura. But if this statement 

 is correct, the Anura in this respect approach the Selachians 

 and Teleosteans, as will be shown further on. 



It is sufficiently clear from what has been stated that in 

 the case of the Amphibia also a distinction must be drawn 

 between the dorsal invagination and the formation of the 

 intestine. They are two different processes, which have a 

 certain relation one to another only because the formation in 

 one organism of two neighbouring organs always shows 

 certain points of contact or connexion. As regards the forma- 

 tion of the mesoderm, this arises from ectoderm as well as 

 from endoderm, and moreover not only in the lateral meso- 

 dermal rudiments to which reference has already been made, 

 but also in the so-called ventral mesoderm, in which also no 

 sharp dividing line is to be drawn between the ectoblasto- 



