GASTRULATION AND EARLIEST DEVELOPMENT 173 



LwOFF's views. — Yet the conception so ably objected to 

 by GOETTE has been formulated once more in a very 

 definite way by LWOFF (1894), who asserts that at what 

 hitherto was known as the gastrulation of Amphioxus not 

 only the entoderm cells invaginate but also part of the 

 ectoderm cells, forming the archenteron roof, the dorsal plate, 

 from which the notochoid and the mesoderm arise. Thus the 

 resulting gastrula-like stage is by no means homologous to 

 the gastrula of Invertebrates. In Amphioxus itself truly the 

 difference between the cells of the supposed ectodermal part 

 of the arc)ienteron-wall and the endodermal part is not 

 so very pronounced and LwOFF states that, had he been 

 dealing with the development of i4/72p/2/oxws alone, he would 

 not have ventured to put forward the hypothesis of an 

 ectodermal origin of the dorsal wall of the archenteron, but 

 that, as he found in other Vertebrates that this dorsal wall 

 was entirely used up in the formation of the notochord and 

 mesoderm, and was in some cases apparently derived from 

 ectoderm, he felt justified in applying this interpretation 

 to the developmental processes of Amphioxus also. 



Some have seen in LwOFF's article the inauguration of 

 a period of better understanding of the early development 

 of Vertebrates, others the beginning of an ever increasing 

 confusion of thoughts. To the former e. g. HUBRECHT 

 belongs. Among the latter may be cited MACBRIDE (1898, 

 p. 597) who writes: "Such an attitude of mind seems to 

 me the entire converse of the proper one to be adopted 

 under these circumstances. Quite apart from the superior 

 value to be attached to the significance of the processes in 

 Amphioxus owing to the primitive nature of the adult, it is 

 one of the best known facts of embryology that the presence 

 of large quantities of yolk clogs and utterly distorts the 

 developmental processes, and that we have to interpret the 

 cases where much yolk is present in the light of those 

 where little yolk is present, and not vice versa. Moreover, 

 a very simple and natural explanation can be suggested 

 why in the Vertebrate embryo the yolk should be confined 

 to the ventral wall of the archenteron. We know that many, if 

 not most, developmental processes are ultimately reducible 

 to processes of folding, such as would be rendered entirely 

 impossible were the tissue in which they have to take 

 place clogged with yolk. Hence in the higher Vertebrates 

 the processes of invagination itself are profoundly modified; 



