138 Gastrulation and Embryo Formation in Amia Calva 
vagination were wholly in the part of the surface layer which belongs to 
the epiblast. A comparison with other forms, however, such as the 
various Amphibia, leads one to hesitate in such an interpretation. The 
erucial factor is the determination of the limits of the ectoblast. If the 
ectoblast be considered as extending to the point where the smaller cells 
pass over into the large volk masses, then the invagination is in the ecto- 
blast. If it does not extend to this point, there are no features which will 
enable us to determine how far it does extend. 
A more highly magnified view of the blastoporic region is shown in 
Fig. 25. The section is taken from an egg of the same age as that shown 
in Fig. 24. In many eggs of this stage, there is a stratum or tongue of 
cells which is somewhat peculiar. This stratum is directly continuous 
with the deep ectoblast at the dorsal lip of the blastopore. Anteriorly its 
cells are separated from the deep ectoblast by the segmentation cavity 
above, while below they pass over into the entoblastic cells. In this stratum 
which is from four to five layers thick, two kinds of cells are present. 
The more numerous are cells which conform in structural peculiarities 
to those of the deep ectoblast. The less numerous are cells which possess 
the structural features of the entoblast. This layer of cells Sobotta, 96, 
has described as mesoblast. Since this layer not only contains mesoblast 
but also entoblast, we have decided to designate the layer as mes-entoblast 
(m. en). 
In Fig. 26 there is represented a meridional section of an egg some- 
what older than that just described, but younger than that shown in sur- 
face view in Fig. 5. The archenteron or gastral cavity is more extended 
and its dorsal wall is formed of cells which are so much like those of the 
superficial ectoblast that one is inclined to regard invagination as still 
playing the more important réle. The rounded cells at the end of the 
gastral cavity are further confirmation. In short it may be said that 
thus far there are no reasons for considering delamination as a factor of 
any importance in the formation of the gastral cavity. 
No particular changes are noticed in the character of the cells in the 
region where the head of the embryo is about to appear. The yolk 
derivatives are widely scattered in the segmentation cavity and many 
cells which, from the character of their granules, would be called yolk 
derivatives are still to be found scattered among the ectoblast cells. The 
large yolk masses are still actively budding off cells and this process has 
gone on so rapidly in this particular egg that these masses have become 
greatly reduced in size. 
Hgg Fifty-three Hours After Fertilization. Blastodise Covers About 
200°. The anlage of the embryo can now be faintly recognized in sur- 
