142 Gastrulation and Embryo Formation in Amia Calva 
which has been derived from the mes-entoderm. We were at first inclined 
to regard these cells as exclusively mesodermal but since they later lose 
their distinctive character the question cannot be definitely settled. 
A sagittal section of an embryo slightly later than the preceding is 
shown in Fig. 33. The principal changes are the further extension of 
the blastodise and the corresponding reduction in the diameter of the 
yolk plug. The peculiar differentiation of the inner layer of the deep 
ectoblast is here prominent. ‘The segmentation cavity is vanishing, the 
gastral cavity enlarging. The yolk is being rapidly segmented, especially 
at its periphery. 
Embryo Sixty-five Hours After Fertilization. Blastodisc Covers 
About 355°. Embryo Extends Over About 140°.—The surface view of 
an egg in this stage is represented by Fig. 9. The embryo is now much 
better defined. The anterior portion is somewhat broader than the trunk, 
which in turn becomes narrowed towards the blastoporic end. The blasto- 
pore is almost closed. In its closure one rarely finds the condition so 
frequently found in the amphibia where the lateral lips approximate so 
much faster than the dorsal and ventral that a slit-like blastopore arises. 
Embryo Seventy Hours After Fertilization. Embryo Extends Over 
About 154°. The next surface view (Fig. 10) represents an embryo 
about five hours older than that shown in Fig. 9. The features noted in 
addition to those described in the preceding stage are the further elon- 
gation of the embryo; the presence of a well-marked neural trench; the 
further closure of the blastopore. At this time there are no external 
evidences of optic vesicles, protovertebre, or pronephric ducts. 
A sagittal section of the posterior portion of an embryo in this stage 
is shown in Fig. 34. At this time the blastopore is nearly closed. The 
external epiblast, as in the earlier stages, is a single layer of cells which 
still retain their peculiar coarse granules and deep staining capacity. 
These cells are in direct continuity with the single layer of cuboidal cells 
lining the blastopore. These cuboidal cells in turn pass over into the 
elongated layer of hypoblastic cells which form the dorsal wall of the 
gastral cavity. The floor of the gastral cavity is made up for the most 
part of a single layer of entodermal cells. The appendicular portion of 
the gut (a. g.) is lined by cells similar to those just described. 
A transverse section through the blastopore of an embryo in the same 
stage is shown in Fig. 35. The relation of the layers is here more clearly 
shown. It will be noted that the section shows especially well the great 
lateral sheets of mesoblast (mes.) 
A median sagittal section of an embryo a few hours later is shown in 
Figs. 36 and 37. The anterior and middle portions are shown in Fig. 36, 
