136 Gastrulation and Embryo Formation in Amia Calva 
stage. In this particular egg the grooves, instead of following meridi- 
onal lines as usual, diverge more widely than those in the eggs shown in 
Figs. 1 and 2. 
Sections show, although none are figured, that the blastodise in this 
stage is made up of eight to ten layers of cells which gradually pass over 
into the yolk derivatives. The outermost layer of the blastodise has un- 
dergone still further modification in that its cells are more elongated, 
closely apposed and more deeply stained. The large yolk masses are 
actively budding off cells not only around the margin of the blastodise but 
also in the central portion of the yolk. The yolk nuclei, which in the 
earlier cleavage stages, were confined to the upper portion of the yolk 
masses are now frequently found more deeply situated. 
Egg Forty Hours After Fertilization. Blastodisc Covers 130°.—A\- 
though the surface of the egg, as shown in Fig 4, presents no features 
worthy of special comment, changes are going on in its interior which 
merit consideration. Ifa meridional section of an egg in approximately 
the same stage (Fig. 23) be examined it will be seen‘that the blastodise 
proper is made up of from ten to twelve layers of cells so closely apposed 
that they make a compact stratum. In addition to these cells, there is a. 
large number of loosely scattered cells which lie in the space which we 
have hitherto designated as the segmentation cavity. These loosely scat- 
tered cells gradually pass over on the one hand into the cells of the 
blastodise proper and on the other into the large yolk masses. The cells 
of the ten or twelve layers forming the upper portion are smaller, more 
uniform, more closely compacted and contain very fine granules as shown 
in the figure; while the loosely scattered yolk cells and those being 
budded off from the large yolk masses are larger, more irregular in out- 
line and contain coarser yolk granules. These two portions cannot be 
considered as sharply marking off ectoblast and entoblast, since one finds 
in the portion which is largely ectoblastic, large cells which are filled 
with coarse granules; and if granules be the criterion for the separation 
of layers these cells must be regarded as recent derivatives from the large 
yolk cells which have wandered up from the lower portion of the blasto- 
disc. If this interpretation be correct, it is a fact of some importance, 
since the part hitherto considered as exclusively ectoblast contains a con- 
siderable number of cells derived from the large yolk masses. 
It has already been pointed out that the outermost layer of the ecto- 
blast (s. ec.) can be readily distinguished from the underlying layers. A 
glance at Fig. 23 shows that in the locality where this layer passes over 
into the large yolk cells there is a marked proliferation of its elements. A 
study of the remaining sections shows that as yet there is no invagination. 
