140 EMBRYOLOGY. 



not yet overgrown by germ-layers ; the place marked by a star, at 

 wbich in the Amphibia the transition from the small-celled layer 

 to the mass of yolk-cells occurs, or the marginal zone of Goette, is 

 comparable to the margin of circumcrescence in meroblastic eggs. 



In the second place, the question arises : How is the middle germ- 

 layer of Vertebrates developed ? The answer is : By a process of 

 foMing similar to that in the case of Amphioxus lanceolatus. This 

 answer is substantiated by the fact that the individual processes in 

 the development of the middle germ-layer may be correlated with 

 corresponding processes in Amphioxus. 



In view of the fundamental importance of the matter, I formulate 

 in a synoptic and precise manner in six paragraphs the points in 

 reference to which it has been possible to establish an agreement in 

 all Vertebrates. 



1. Before the chorda is formed, the germ in all Vertebrates is 

 composed of two layers in the region of a mediae streak which lies 

 in front of the blastopore and primitive groove. It is here composed 

 of the medullary plate and the fundament of the chorda, which then 

 shares in bounding the intestinal cavity. 



2. At both sides of this median streak the germ is three-layered, 

 if we regard the middle germ-layer as a single one ; it is four-layered, 

 if we allow that the latter consists of a parietal and a visceral cell- 

 layer, which are originally pressed firmly together, and only later 

 actually separated by the appearance of the body-cavity. 



3. In no Vertebrate do the middle germ-layers arise by fission, 

 either from the outer or the inner germ-layers, because they are 

 everywhere, except in a very hmited region of the germ, sharply 

 separa-fced from both by means of a fissure. 



4. A connection of the middle germ-layers with the neighbouring 

 cell-layers takes place only : (a) at the blastopore or primitive groove, 

 where all four (or three) germ-layers are joined together, and (6) at 

 both sides of the fundament of the chorda. 



5. One observes the first fundament of the middle germ-layers at 

 the region of the germ just mentioned, and sees it spread itself out 

 from here — •■i.e., from the periphery of the blastopore or the primitive 

 groove, and from both sides of the fundament of the chorda 

 — forward, backward, and ventrad or laterad. In front of the 

 blastopore it appears in the form of paired fundaments separated by 

 the fundament of the chorda ; behind the blastopore, on the contrary, 

 as a continuous structure. 



6. While the chorda is being developed, the two paired fundaments 



.j^ 



