144 Gastrulation and Embryo Formation in Amia Calva 
Another section of the same embryo through the posterior portion of 
the hind brain is represented in Fig. 39. On either side of the neural 
keel and in close proximity to its dorso-lateral margin, there are deeply 
staining groups of cells which are probably spinal ganglia; although it 
should be said that in some preparations they appear to be proliferations 
of the inner layer of the superficial ectoblast. The notochord (ne.) is 
well differentiated at this level. Beneath it the layers forming the walls 
of the gut are in contact so that the lumen is here obscured. On either 
side of the notochord, however, the layers separate and the laterally ex- 
tending gut cavity (g. c.) is obvious. 
Another transverse section at the level of Kupffer’s vesicle is repre- 
sented in Fig. 40. In the median line there is a groove in the superficial 
epiblast (n. ¢.) which we have interpreted as a neural trench. It ex- 
tends backward to the point where the scattered coarsely granular cells 
indicate the line of closure of the blastopore (cf. Fig. 37). The deeper 
epiblast has not yet taken on the form of a neural keel, but extends lat- 
erally to a considerable distance. ‘The notochord is well differentiated 
and consists of cells whose character lends confirmation to the view that 
they are derived from the mesoderm rather than the gut hypoblast. At 
any rate we have not observed the coarsely granular cells of the hypo- 
blast participating in its formation. 
The section represented by Fig. 41 is taken through the posterior por- 
tion of an embryo of about the same age. In this embryo a deeper neural 
trench (n. t.) is present than in the preceding. The posterior end of the 
notochord, as it passes over into the mass of undifferentiated cells is 
barely defined by the peculiar arrangement of its cells. Kupffer’s vesicle 
is smaller than in the preceding embryo. In this structure there are wide 
variations in size as may be inferred by glancing at the different figures. 
Embryo Bighty Hours After Fertilization. Embryo Covers About 
160°.—The surface views (Figs. 13 and 14) show that the embryo is 
considerably advanced beyond the stage represented in Figs. 11 and 12. 
The body of the embryo is narrower; the optic vesicles are more promi- 
nent; seven to nine protovertebre are differentiated; the pronephric 
ducts are forming. In the anterior portion of the embryo there are 
three fairly well defined regions which represent the primary divisions 
of the brain. Anterior to the optic vesicles the nervous system is con- 
tinued into a conical process, the homologue of the structure which Sa- 
lensky found in Acipenser and to which he gave the name “ Stirnforsatz.” 
This precerebral portion of the head is the anlage of several structures 
to which we shall hereafter refer in greater detail. The mid-brain is 
marked off by a constriction posteriorly and behind this constriction is a 
