14 J. T. CUNNINGHAM. 
of the invaginated layer or ring, but their attention is chiefly 
devoted to the method of its formation, which is now to be 
considered more particularly. 
Haekel’s view (4) was that the invaginated layer was pro- 
duced simply by the multiplication of the cells at the edge of 
the blastoderm, the new cells arranging themselves as a single 
layer, which insinuated itself between the blastoderm and the 
yolk. Ryder, as has been mentioned above, believes that there 
is no ingrowth at all from the edge of the blastoderm, but that 
after the appearance of the segmentation cavity the cells of the 
blastoderm split upin sitdi into three layers, the lowest of 
which, next to the periblast, comprises’ hypoblast and meso- 
blast ; the next above is the sensory layer of the epiblast, and 
the outermost is the epithelial or epidermic layer. Kingsley 
and Conn formulate very distinctly the conclusion, supported 
by detailed evidence, that in the ovum of Ctenolabrus 
ceruleus the invaginated layer is produced, as Haeckel be- 
lieved, by the gradual ingrowth of the edge of the outermost 
layer of the blastoderm. According to these observers the 
layer consists of only a single stratum of cells. Agassiz and 
Whitman are also of opinion that the invaginated layer arises 
by ingrowth from the edge of the blastoderm; ‘though, of 
course,” they say, “ there is no such wholesale invagination as 
supposed by Haeckel.” This means, apparently, that the in- 
vaginated layer does not, as Haeckel supposed, extend com- 
pletely beneath the segmentation cavity. These authors regard 
the centrepetal multiplication of periblast nuclei beneath the 
blastoderm as part of the process of invagination. They also 
make two statements which require to be considered : firstly, 
that the invaginated layer is continuous at the edge of the 
blastoderm, not with its outermost layer, but with the layers 
beneath ; and secondly, that the invaginated layer is a single 
stratum in all regions of the ring except beneath the axis of 
the embryonic rudiment, where it is two to four cells deep. 
The former of these statements cannot be regarded as proved 
by the figure of a prepared section, to which the authors refer 
as evidence. The interpretation of the second statement is 
