122 
size. We may distinguish a central group of large, yolk-laden, 
inactive cells and a peripheral ring of smaller and more 
active entoderm-cells which, by their proliferation and 
moving inwards, contribute especially to the invagination 
and the formation of the archenteric cavity. The wall 
of this cavity in Amphioxus consists partly of the 
larger entoderm-cells and in Amphibians at the corres- 
ponding place it is thickened very much by the presence of 
the mass of yolk-cells which as a voluminous plug project 
into the archenteron, often filling it up for the greater part 
and leaving only a ring-shaped lumen. The ring of smaller 
entoderm-cells is best developed near the anterior border 
of the entoderm-area and as a consequence the archenteric 
cavity at the close of gastrulation is best developed under 
the dorsal blastopore border. This causes the plug of 
yolk-cells to lie not in the centre of the archenteric wall, 
opposite the blastopore, but more to the ventral side. The 
dorsal wall then is formed especially by the smaller ento- 
derm-cells, which contain much less yolk and from which 
afterwards the notochord and the mesoderm are derived. 
In Amphioxus the difference in size of the dorsal and the 
ventral entoderm-cells is insignificant, in Amphibians it is 
already greater and it becomes very considerable in the 
yolk-laden eggs of Selachians and Amniotes. Here as a 
matter of fact the cells of the dorsal archenteron wall by 
their size and appearance stand much nearer to the ecto- 
_derm-cells than to the often enormous yolk laden ventral 
entoderm-cells, and this will easily give rise to the impression 
that an invagination of ectoderm-cells has occurred round 
the dorsal border of the blastopore. These cells would 
have formed the dorsal archenteron wall from which the 
notochord and the mesoderm will be derived afterwards. 
This view has been advocated indeed as early as 187 
by SCOTT and OSBORNE (1879) in their study on the 
development of the newt, and in 1882 and ’83 O. HERTWIG 
came to similar conclusions. The latter identifies the piS- 
mented animal half of the frogs egg with the ectoblast, 
the unpigmented vegetative half with the entoblast. During 
gastrulation the “animal” cells at the dorsal border invag!- 
nate in such a way that the median band of the archen- 
teron roof, from which the notochord originates, is formed 
by them (p. 262-264). The mesoblast-bands are also derived 
from the animal cells (p. 263). 
