RELATIONS OF YOLK TO GASTRULA IN TELEOSTEANS. 15 
probably that beneath the axis of the embryonic rudiment the 
hypoblast is continuous with cells destined to form the 
notochord. 1 
The evidence of the actual ingrowth of a cell layer from the 
edge of the blastoderm is not very clear in the case of the salmon, 
to judge from the elaborate investigations concerning that form 
which are described in published memoirs (12). But it is 
certain that in ova of the Salmonide the general relations of 
the embryonic rudiment, germinal ring, and segmentation 
cavity are the same as in pelagic ova, and the lowest cells of 
the thickened rim of the blastoderm are continuous at the edge 
with the epiblast, although there is no single stratum distinctly 
marked off as an invaginated layer. It is possible that the 
layer in question, the lowest layer of the germinal ring, may 
vary in its mode of origin ; in pelagic ova, such as Ctenolabrus, 
cod, and haddock arising chiefly by centripetal multiplication 
of cells, in Salmonidz to a great extent by delamination from 
the blastoderm. ‘The latter mode of formation can easily be 
explained by the familiar principle of abbreviation of develop- 
ment, according to which cells attain a position ontogenetically 
during segmentation, which phylogenetically was only reached 
by actual change of position. 
A third and quite novel view concerning the origin of the 
invaginated layer has been recently brought forward by George 
Brook (13), who believes that in Trachinus vipera the layer 
is produced by the formation of distinct cells from the nucleated 
protoplasm of the periblast. I am not able to subject this 
proposition to criticism any further than to say that the 
appearances in the living ova of the haddock and cod are best 
explained in the view that the layer in question arises by a 
centripetal ingrowth of cells from the edge of the blastoderm. 
In whatever way the invaginated layer is produced there is 
no uncertainty concerning the main relations of the parts in 
the ova of the cod and haddock at the stage represented in figs. 
5,16, and 17. The yolk is a nearly spherical mass, of which 
that part beneath the blastoderm and a little beyond its edge 
is provided with a layer of nucleated protoplasm—the periblast. 
