THE EARLY DEVELOPMENT OF AMPHIOXUS. 597 



sections, though I do not admit that such an accumulation, 

 did it exist, would prove his point. 



If we examine a transverse section of a completed gastrula — 

 such a one, for instance, as PI. 43, fig. 11 — we find no diflference 

 in character between the cells forming the dorsal wall of the 

 alimentary canal and those forming the ventral wall, such as 

 we should have the right to expect did Lwoff' s hypothesis in 

 any way correspond with the facts. Before leaving this 

 subject, however, it is but just to notice a statement of 

 Lwoff' s, that had he been dealing with the development of 

 Amphioxus alone, he should 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 did not take part in 

 the definitive wall of the alimentary canal, and was in some 

 cases apparently derived from ectoderm, he felt justified in 

 reading this interpretation into the developmental processes 

 of Amphioxus. 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 significauce 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 develop- 

 mental 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 manv, if not 

 most, developmental processes are ultimately reducible to 

 processes of folding, such as would be rendered entirely im- 

 possible were the tissue in which they have to take place 

 clogged with yolk. Hence in the higher Vertebrates the pro- 

 cesses of invagination itself are profoundly modified; and, as 

 explained in detail in the careful work of Will (12) (who in 



VOL. 40, PART 4. NEV7 SEE. T T 



