RELATIONS OF YOLK TO GASTRULA IN TELEOSTEANS. 19 
cell. He regarded the yolk as a quantity of passive material 
which was contained in the gastrula cavity, projected from the 
blastopore, and was ultimately enveloped entirely by the hypo- 
blast. But since the egg before segmentation isa single cell, 
no portion of it can be separated from the rest except by a 
process of cell-division, and any such separated portion must 
have the value either of a cell or a number of cells. It remains 
true, as shown below, that the yolk projects from the blasto- 
pore; but the relations of yolk and blastopore to one another 
and to the embryo were not by any means completely or cor- 
rectly stated in Haeckel’s account. These relations need to be 
examined again in the light of the additional knowledge which 
has been gained since the period of the ‘‘ Gastraea-theorie.” 
The facts from which an inquiry into the ancestral origin of 
the processes of Teleostean development must start are— 
firstly, that in the actual development of many animal forms 
the primitive digestive cavity is formed by an invagination of 
part of the wall in a hollow spherical body; secondly, that in 
Elasmobranchii and Amphibia an inflection of, or growth in- 
wards from, the edge of the blastoderm forms a layer beneath 
the axis of the embryonic rudiment, which layer becomes 
without material alterations in its constitution the dorsal wall 
of the permanent intestine. 
The broad proposition that this process of inflection in 
Elasmobranchs and Amphibians represents the primitive pro- 
cess of invagination in a simple gastrula is universally accepted 
hy embryologists. There is still a good deal of obscurity 
about the history of the successive changes in the development 
of the embryo by which the modified invagination in the two 
types mentioned was derived from the primitive invagination 
in a typical gastrula. 
Without any comparison with other types it is an unavoidable 
conclusion that the inflection of the edge of the blastoderm, 
so conspicuous in the eggs of the cod and haddock, and other 
pelagic Teleostean ova, represents in some degree the invagina- 
tion of the ancestral gastrula. But in the embryos of Elasmo- 
branchs and Amphibians the inflected layer becomes the dorsal 
