200 
J. D. F.- GILCHRIST. 
this substance. The cavity as seen in sections is usually 
rounded or oval in shape, but that it is in reality of an elongate 
nature is evident from the fact that it can be followed 
through a series of consecutive sections. It ends abruptly 
when traced in one direction, but may be followed in the 
Other direction to the periphery of the embryo, where it 
suddenly ends in a shallow pit in the ectoderm, apparently 
the involution or invagination of the ectoderm already noted. 
This was most clearly seen in sections which passed through 
the axis of this part of the embryo (PI. 13, fig. 16). The 
whole assumed the form of a structure, which, without this 
explanation of its origin, might be taken to be a typical 
gastrula, in which the central yolk-laden mass represents an 
endoderm, formed by invagination, and a central cavity, the 
archenteron ; the only suspicious feature being the very 
narrow lumen and the absence of cellular structure of the 
endoderm. 
Andersson (2, p. 87) was the first to notice and figure this 
gastrula, and he has apparently no doubt as to how it has 
arisen. He notes HarmePs suggestion ( 5 , pp. 109, 110) that 
the gastrula is probably formed by a process of delamina- 
tion, and considers that for his species at least this is not the 
case, but that “ die Gastrula durch eine typisclie Invagina- 
tion sicli bildet 99 (p. 89). He was unfortunately unable, lie 
adds, to carry out any study of the cellular structure, as 
owing, he believes, to imperfect preservation, the ectoderm and 
endoderm appeared uniformly filled with yolk granules, which 
he notes, however, were absent in the immediate vicinity of 
the central lumen. His figure, however, indicates the existence 
of the external yolk columns. Schepotieff (8, p. 437) states 
that he found gastrula stages in C. indicus, but was unable 
to follow out their formation. That, however, he accepts the 
view that the central yolk mass represents an endoderm 
formed by invagination, is apparent from his description and 
figures of larval stages of the species. Harmer also 
( 4 , p. 245) accepts the view that the central yolk represents 
the wall of the archenteron. 
