30 ART. 11. — 8. HATTA. 



by it« backward shifting and superficial extension. In consequence 

 of these processes, the large arched blastopore suffers changes in its 

 shape and ultimately acquires the definitive form. After all, the 

 whole occurrences are nothing else than those which answer to 

 the overgrowth of the animal layer over the large blastopore in 

 Amjyhioxus in which it takes place only after complete invagi- 

 nation of the vegetative hemisphere, while here in Fetromyzon 

 earlier phases of the overgrowth overlap to some extent later 

 phases of the invagination. At any rate, it is true that some 

 later phases of the gastrulation in Fetromyzon as well as in 

 Ainphloxus, consist entirely in overgrowth of the micromeric 

 epithelium. 



The invaginated micromeric epithelium is not of a great 

 breadth from side to side ; however, it occupies completely the 

 roof of the archenteroii whicli presents, on cross-sections, a 

 small lentiform shape, while the lateral walls and the fioor of 

 the cavity are formed by the macromeric cell-group {Fiys. 24 

 and 29). It is plain that this relation of the micromeric 

 epithelium to other parts has been brought about in such a way 

 that the macromeric epithelium (folded at the blastoporic lip) 

 partly migrated inwards and partly caused the lip to grow farther 

 backwards, while the depressed macromeric surface was deeply 

 withdrawn leaving a concavity on the surface. The archenteric 

 cavity is accordingly represented by the depressed or grooved surface 

 of the macromeric hemisphere itself (compare Fly. 29 with Fiy. 28). 

 It is this part of the micromeric epithelium which is called by 

 Lwoff^^ the " Dorsalplatte." It must be remarked here that the 

 boundary between the micromeric " dorsal plate " and the macro- 

 meric lateral walls and floor however does^ not correspond to the 



1) loc. cit. 



