RELATIONS OF YOLK TO GASTRULA IN TELEOSTEANS. 7 
there is a nucleus. By the continued repetition of this seg- 
mentation, which soon begins to take place also in planes 
parallel to the surface, the blastodisc is converted into a mass 
of nucleated cells, forming the blastoderm. While the cells 
are still large enough to be distinguished by a power of forty 
diameters the blastoderm in the ova of the cod, haddock, and 
whiting is a prominent hemispherical projection having a 
slightly convex base, which rests in a shallow concavity of the 
yolk. The appearance of the living ovum of the haddock at 
this stage is shown in fig. 2, which represents an optical section 
in a plane dividing the ovum symmetrically. Fig. 11 represents 
the corresponding stage in the ovum of the cod observed in a 
similar position. If a view is obtained of a part of the surface 
of the ovum, which includes part of the edge of the blastoderm, 
it is seen that the boundary between the latter and the yolk is 
not sharply defined; that the cells at the edge of the blasto- 
derm have no planes of division separating them from the yolk- 
mass, though their nuclei and the planes of division separating 
them from one another and from the other cells of the blasto- 
derm can be distinctly seen. The appearance referred to is 
shown in fig. 12 as seen in the ovum of the cod. Agassiz and 
Whitman (1) have ascertained from sections of the ovum of 
Ctenolabrus that the cells at the edge of the blastoderm are at 
the sixteen-cell stage, and from that period up to the period of 
invagination continuous with a layer of protoplasm, which 
extends over the yolk outwards beyond the edge of the blasto- 
derm, and inwards beneath the blastoderm. According to 
their results the blastoderm is connected with the yolk only by 
the circular series of cells at its edge, the rest of it being 
separated from the yolk by the segmentation cavity, which is 
present as early as the sixteen-cell stage, but remains for some 
time very small. 
The state of things described above, as seen in the living 
ovum of the cod, agrees perfectly with the results of Agassiz 
and Whitman. By careful focussing the protoplasm covering 
the yolk in the neighbourhood of the blastoderm can be seen 
at this and earlier stages of segmentation in optical sections, 
