374 HeiT B. Lwoff on the Formation of the 



In no Vertebrate is tlie intestine formed by invagination. 

 The cnduderm cells are overgrown by those of the ectoderm, 

 and the intestine arises through divergence of the former. In 

 the process of overgrowth a modified gastrulation may be seen. 

 But the further this process extends tlie more it is suppressed. 

 In merobhistic ova, where the yolk is very large (Selachians, 

 Sauropsida), the process of overgrowth is so greatly modified 

 in consequence of adaptation to the new conditions that 

 gastrulation is practically entirely suppressed. The dorsal 

 ectodermal invagination, on the contrary, exists in all Verte- 

 brates as a cJiaracteristic process, which has nothing to do 

 with the formation of the intestine, and is to be regarded as 

 a new formation *, that is, as the ingrowth of the ectoblas- 

 togenous rudiment of the notocliord and mesoderm. This 

 rudlmejit develops from the cells of the ectoderm and has a 

 common origin with the rudiment of the nervous system. 



(3) In no Vertebrate is a true gastrula to be observed ; it 

 is therefore impracticable to search for the gastrula mouth in 

 its proper sense, and to endeavour to discover its dorsal and 

 ventral lips, since the primitive conditions are practically 

 entirely obliterated by the new formations. Accordingly I 

 leave this question out of consideration. I consider all 

 attempts to homologlze the dorsal and ventral lips of the 

 gastrula mouth in different Vertebrates as forced and un- 

 natural, 



(4) There is only one spot in the gei-m the homology of 

 ■which can be traced in all Vertebrates, namely that from 

 which the invagination f of the ectoderm cells proceeds — 

 in other words, where the ingrowth of the ectoblastogenous 

 rudiment of the notocliord and mesoderm commences. 



(5) The so-called neurenteric canal appears to me in a very 

 difierent light. It is not a tube of communication between 

 the nervous system and the intestinal tract. Just as the 

 margin of the enveloping layer represents a transitional region 

 between the cells of the rudiment of the nervous system and 

 the rudiment of the notochord, so this canal is to be regarded 

 as the morphological expression of this transition. There- 

 fore it may be regarded as a canal of communication between 

 the tube of the nervous system and the notochordal groove or 

 notochordal cavity, and may be termed the neurochordal 



* It will be evident from this that I virtually dissent from the cus- 

 tomary view,_according to which the posterior end of the germ, where the 

 invagination is to be observed, exhibits more primitive conditions. 



t In the majority of cases it is really not a true invagination, but 

 merely the ingrowth of the cells or the turning-in of the margin of the 

 enveloping layer. 



