12 J. T. CUNNINGHAM. 
In the memoir already referred to Ryder gives two figures 
(3, figs. 15 and 16) illustrating the period of development now 
in question, as studied by him in the cod. One of these figures 
(loc. cit., fig. 15), representing an optical section, indicates a 
state of things which, if my present views are well founded, is 
impossible. According to this figure the segmentation cavity is 
completely bounded below by a layer of ‘ hypoblast,” continu- 
ous, apparently, with what I have called the invaginated layer ; 
but the limits of the invaginated iayer, clear and distinct as they 
are in the living ovum, are not indicated in Ryder’s figure. 
I have never seen any stage in which the segmentation cavity 
was bounded inferiorly by a layer of cells or by anything but 
the surface of the yolk (periblast) ; and if there be such a stage 
at all it certainly does not occur after the appearance of the 
invaginated layer; but Ryder’s figure evidently belongs to the 
same stage as that represented by Nos. 5 and 16 of my figures, 
and therefore if my figures are correct his is erroneous. 
In the text of his paper (p. 88) Ryder contradicts the pro- 
position implied in his figure, that the segmentation cavity is 
bounded inferiorly by a stratum of distinct cells, and says that 
its floor is formed “ by the yolk hypoblast, which is not truly 
hypoblastic, and which corresponds to the granular layer of 
Balfour.’ It is difficult to form a clear conception from Ryder’s 
paper of what he has seen of the relations of the primary layers 
at this period of Teleostean development, or what conclusions 
he has drawn. The figures he gives of the relations of parts at 
succeeding stages agree closely with my own results, but all 
that he says distinctly concerning the early condition of the 
invaginated layer is that it arises by delamination from the 
blastoderm. 
There are two subjects of inquiry connected with the seg- 
mentation cavity and the invaginated layer, which in Ryder’s 
pages are continually confused. Firstly, what are their rela- 
tions with other parts of the blastoderm at successive stages ? 
secondly, what are the processes by which those relations are 
produced ? 
Ryder refers to Haeckel’s account of the development in a 
