HISTORY OF THE GERM-LAYER THEORY. 151 



Vertebrates, arise, not through disassociation or fission, but through 

 infolding of an originally simple cell-layer.* (2) These are com- 

 parable with one another or homologous, because they are developed 

 according to the same process, and because the two fundamental 

 organs of the body, the layer which limits the body externally 

 {the ectoderm) and the layer which lines the digestive cavity (the 

 ^nfcoderm), arise from them. (3) The intestinal canal of all animals 

 arises by invagination. 



In the question as to the development of the middle germ-layer 

 HAECKEL remained at the traditional standpoint, and inclined most 

 to C. E. VON BAER'S view that the parietal lamella arose by fission 

 from the outer primary layer, and the visceral lamella from the 

 inner germ-layer. Most embryologists, who worked on the develop- 

 ment of Vertebrates, entertained, on the contrary, REMAK'S view, 

 -and made the whole middle germ-layer arise from the inner 

 by fission. 



They regarded the body-cavity as a fissure in the middle germ- 

 layer, and compared it with other lymphatic spaces, such as occur in 

 the connective tissue at various places in the body. 



The correction of this view was undertaken by various persons 

 in the same manner as in the case of the primary germ-layers. By 

 detailed study of the formation of the germ-layers in the Chick 

 and Mammals, KOLLIKER found that the middle germ-layer did not 

 simply split itself off from the inner, but that it arose from a limited 

 region of the blastoderm, namely, from the primitive groove, where 

 the two primary germ -layers are continuous. He maintained that 

 from this region it grew out between the two primary germ-layers 

 as a solid cell-mass, and that subsequently the body-cavity appeared 

 in it by means of its fission into two layers. This was an essential 

 advance in the representation of the actual state of affairs. 



But a deeper insight into these embryonic processes in Vertebrates 

 was first acquired in this case also through the study of Invertebrates, 

 especially through the important discoveries of METSCHNIKOFF and 

 KOWALEVSKY concerning the formation of the body-cavity in Echino- 

 derms, Balanoglossus, Chsetognathi, Brachiopods, and Amphioxus. 

 The former found that in the larvae of Echinoderms and in Tornaria, 

 the larva of Balanoglossus, the walls of the body-cavity are formed 

 from evaginations of the intestinal canal. But a still greater sensation 



* It is still affirmed by several authors for certain Invertebrates that the 

 inner germ-layer develops, not by infolding, but by a splitting off or delamina- 

 tion from the outer germ-layer. 



