124 
LWOFF’s views. — Yet the conception so ably objected to 
by GOETTE has been formulated once more in a ver 
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 notochord 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 archenteron-wall and the endodermal part is not 
so very pronounced and LWOFF states that, had he been 
dealing with the development of Amphioxus 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. 
ome 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 Ill 
Amphioxus owing to the primitive nature of the adult, itis 
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 versâ. 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; 
