THE DEVELOPMENT OE THE GAPE CEPHALODISCUS. 203 
obscured by the compression of the embryo in a small space, 
become clearer or assume a different aspect in the early larva. 
Thus the ectoderm, very much folded in the embryo, now 
expands, and the body cavities can more readily be made out. 
Certain more definite changes, however, were observed in 
older larvae. 
The general structure of the larva has already been 
described by Harmer (5), Andersson (2), and Schepotieff (8). 
The chief new points to be added are in connection with (1) 
the fate of the internal yolk mass, (2) the arrangement of the 
body cavities and the mode of origin of the anterior body 
cavity, (3) the origin of the anus, (4) the involution of the 
sense organ and its nervous tissue, and (5) a postero-ventral 
thickening and involution. 
(1) Fate of yolk mass. — Neither Harmer nor Andersson 
found any trace of cellular structure in the yolk. Schepotieff, 
however, indicates clearly ( 8 , PL 8 fig. 16) that this part 
is divided up into large columnar cells, which, but for their 
distinct demarcation and single nuclei, might pass for the 
internal yolk columns, already described for the species 
under consideration. His fig. 7 also shows the walls of what 
he regards as the proboscis-ccelom, ending abruptly at the 
anterior part of the yolk, instead of passing round it to the 
posterior extremity, as here described. The cells of this 
body cavity are obviously very diagrammatically drawn, and 
it may be that those of the “Urdarm” in fig. 16 are of 
the same nature, in which case it would not be so difficult to 
interpret them as internal yolk columns. It is not, however, 
absolutely necessary to reconcile other accounts of the for- 
mation of the endoderm with that given here, as both may 
be correct; the mode of development, even in closely related 
forms of animals, having been proved in some cases to be 
very different. 
The further changes observed in the central yolk mass were 
as follows : The anterior space (archenteron) between the 
endoderm and the yolk becomes very large, and at the same 
time the yolk lumen increases in size (PI. 13, fig. 19, y. L). 
