472 
The An 
Naturalist. 
[June, 
The regularity of the cleavage cannot be followed further, but 
the upper pole continues to undergo a more rapid segmentation 
than the lower. At the close of segmentation the &z<g forms a 
sphere, containing an excentric segmentation cavity (Fig. 4, 
s. c) composed of two unequal part's, an upper arch of several 
layers of cells, {Bl.) 
the primitive blasto- 
derm of Minot or ecto- 
derm, and a lower 
mass {Yolk) of large 
cells rich in proto- 
plasm. At the edge 
of the mass of large 
cells {k w) there is a 
gradual passage in size 
to the cells of the 
blastoderm, and it ap- 
that the small 
sceive additions 
at the expense of the 
large ones ; this zone 
corresponds to the so- 
called germinal wall of large vertebrate ova, and also to what 
we have defined as the ectental line. 
The secondary type of vertebrate segmentation dififers from the 
primary principally in the retarded development of the ento- 
derm, due, apparently, to the increase of the yolk-matter. The 
yolk granules are, as already mentioned, found to be situated 
not quite exclusively, though almost so, in those parts of the 
ovum out of which the entodermal cells are formed. Hence, 
when there is a great deal of yolk the anlage of the entoderm 
becomes bulky, and when it segments the entodermal cells it 
produces are correspondingly big, as we have seen is the case 
in Amphibian ova. On the other hand, when the amount of 
yolk is small, as in the primitive type of segmentation, e. g. 
echinoderms, the entodermal cells are small. In the reverse 
case when the amount of yolk is exceedingly great, as in se- 
