BI0TOB1 or i in (.I:I;M LAI IB NII:>UY. 1 SI 



Vertebrate^. ari-e, not through di>as-M;-iatii>n or ti-Moii. but t!i rough 



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



paiable with one another or homolo- TOM th<-y are developed 



:din t ij to the same process, and because the two fundamental 



us of the body, the layer wiiidi limits {}*< nally 



. ct dfMii) and the layer which lines the ili.ijrst ive cavity (the 



1, run, ari-e from them. (3) Th- intestinal canal of all animals 



UJaee by invairinaiioii. 



In the (jue>tion a> to the development of the middle germ-layer 

 11 AI:I KI:I, remained at the traditional standpoint, and inclined most 

 K. VON BAER'S view that the parietal lamella ane by fiion 

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

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

 ment of Vertebrates, entertained, on the contrary, HI:MAK'< new, 

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

 by tission. 



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 per- 

 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 

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

 the two primary germ -lay ers are continuous. Jle 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 pi n Vertebrates 



was ; i red in t his case also through the study of Invertebrates, 



ially through the important discover h i-< BHIKOFT and 



K'I\V.M.KVSK\ concerning the formation of the body-i a\ it \ in K< -hino- 

 derms, Balanoglossus, Chretognathi, Brachiopods. and Amphioxuv 

 The former found that in the larvae of Kcbinoderm> and in Toruaiia, 

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

 from evaginations of the intestinal canal. J'.ut a still greater sensation 



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

 inner germ-layer develops, not by infolding, but :ing off ordelaminn- 



tion from the outer germ-layer. 



