RELATIONS OF YOLK TO GASTRULA IN TELEOSTEANS. 15 
pelagic Teleostean ovum (4), in order to contradict several of 
the propositions contained therein; but the propositions of 
Haeckel which he refutes are not those which are altogether 
erroneous. Ryder says that Haeckel is wrong when he implies 
that the whole under surface of the blastoderm is lifted up from 
the yolk, and remains in contact with the latter only round its 
margin ; and yet two pages before he expresses agreement with 
Klein in the view that the segmentation cavity is formed by the 
elevation of the blastoderm, so that it is freed from contact with 
the parablast layer lying just below it. The second proposition 
of Haeckel’s which Ryder disputes is that the hypoblast is 
formed by a reflection inwards of the margin of the blastoderm ; 
and, thirdly, he denies the statement of Haeckel, that the seg- 
mentation cavity disappears. 
With regard to this last point it is certainly true that in the 
cod and haddock, and probably in all pelagic Teleostean ova, 
the segmentation cavity is not obliterated, as Haeckel taught, 
immediately on the formation of the invaginated layer, by that 
layer growing inwards till it forms a complete stratum beneath 
the blastoderm. The segmentation cavity persists, as will be 
seen below, till the yolk is almost completely enveloped by the 
blastoderm ; but whether it persists in the adult, as Ryder sup- 
poses, is another question. According to my own observations, 
Haeckel was right in his account of the formation of the seg- 
mentation cavity, and of the earliest relations of the invagi- 
nated layer, except that he was not aware of the existence of 
the periblast. 
The relations of the invaginated layer at the earlier stages 
of its existence have been correctly represented by Kingsley 
and Conn, as seen in the ovum of Ctenolabrus ceruleus, 
in figs. 22 and 23 of their memoir (2). The relations of the 
segmentation cavity at later stages are not clearly indicated by 
these authors. 
As far as can be judged from their paper the conclusions of 
Agassiz and Whitman (1), whose observations were made on 
Ctenolabrus, Pseudorhombus melanogaster, Ps. ob- 
longus and Tautoga, agree with my own as to the relations 
