366 HciT B. Lwoff o?i the Formation of the 



endoderm cells which subsequentlj form the intesfine are not 

 invaginated ; they only experience certain displaceinents in 

 consequence of the dorsal invagination, whereby the segmen- 

 tation cavity becomes obliterated. Elements alone are 

 invaginated from which the notochord and the mesoderm 

 arise, and moreover t])is invagination takes place in such a 

 way that tlie ectoderm cells grow inwards from the margin of 

 the enveloping layer and form the continuous dorsal plate, 

 the ectoblastogenous rudiment of the notochord and mesoderm. 

 1 see no reason for designating the cells of the dorsal wall of 

 the cavity as endodermal, merely because they grow inwards 

 (become invaginated). Similarly I see no reason for terming 

 the cavity at this stage the gastrula or archenteric cavity. 

 At this stage an enteric cavity is not yet present : it is con- 

 stituted subsequently, when the endoderm cells diverge one 

 from another and form the epithelial wall. 



From the central portion of the dorsal plate is differentiated 

 the rudiment of the notochord ; the two lateral portions, 

 together with the adjoining cells of the endoderm, form the 

 rudiments of the mesoderm. As a rule no sharp dividing line 

 can be drawn between the ectoblastogenous and endoblasto- 

 genous cells of the mesoderm, for the cells which are invagi- 

 nated grow inwards, while they closely adjoin the cells of the 

 endoderm. After the rudiment of the notochord has separated 

 off from the lateral mesodermal rudiments, the mai'gins of the 

 endoderm grow towards one another, to close the enteric 

 wall ; but before this takes place the rudiment of the noto- 

 chord is temporarily included in the dorsal wall of the intes- 

 tine. Since the margins of the endoderm continue to grow 

 beneath the notochord, the latter is excluded, the edges of the 

 endoderm unite, and the enteric cavity closed on all sides is 

 now constituted. 



1 was able to observe the same developmental processes in 

 the case of Aocolotl also. In this case certain points tell even 

 more in favour of my conception than in Petromyzon, While 

 the larger endoderm cells are being overgrown by the smaller 

 ones of the ectoderm the dorsal invagination begins. This 

 invagination is nothing else than the ingrowth of the ecto- 

 derm cells, which grow inwards from the margin of the enve- 

 lope and form the continuous dorsal plate of cells which, as 

 in the case of Petromyzon^ represents the ectoblastogenous 

 rudiment of the notochord and of the mesoderm. This 

 invagination in no way assists in the formation of the enteric 



