16 J. T. CUNNINGHAM. 
The blastoderm has a circular form, and beneath its edge is a 
layer of cells in the form of a flat ring, whose lower side is in 
contact with the periblast. In the neighbourhood of one of its 
radii the blastoderm is much thicker than elsewhere, and this 
thickening forms the embryonic rudiment, beneath which the 
invaginated layer extends further inwards than at other parts 
of the ring. Where the invaginated layer is absent there is a 
cavity separating the more central part of the blastoderm from 
the periblast—the segmentation cavity. It is a conspicuous and 
important fact that the inner edge of the invaginated layer is 
distinct and sharply defined, and that there is apparently no 
direct continuity between this edge and the periblast on which 
it rests. The significance of this fact will be considered 
subsequently. 
Growth of the Blastoderm round the Yolk. 
Successive stages in this process, as seen in the living ovum 
of the haddock, are represented in figs. 6—9 ; fig. 18 shows a late 
stage in the process in the whiting ; figs. 20, 21, and 22 repre- 
sent successive stages in the herring ovum. Some stages of the 
process in the cod ovum are correctly represented by the figures 
of Ryder ; in Ctenolabrus by the figures of Kingsley and Conn. 
The main features of the process as seen in the haddock and 
cod are that the embryonic rudiment grows in length with the 
growth of the blastoderm, its posterior end always remaining at 
the edge of the latter; that the segmentation cavity increases 
greatly in extent, at the same time its depth decreases, and it 
becomes very thin; that the part of the embryonic ring out- 
side the embryonic rudiment decreases slightly in breadth, and 
that the ring gradually contracts as it reaches the lower pole 
of the ovum, and diminishes to a small opening on the 
posterior end of the dorsal surface of the embryo, and this 
opening finally closes. The most important fact about the 
envelopment of the yolk by the blastoderm is that the embryonic 
ring is gradually absorbed by the increasing embryonic rudi- 
ment until the whole of it forms part of the dorsal regicn of 
the embryo. As this process of absorption goes on the 
