146 Gastrulation and Embryo Formation in Amia Calva 
and become continuous with the yolk. In the figure given the dorsal 
hypoblast has been too deeply shaded so that it is brought out in too 
strong contrast with the layer of yolk-bearing cells which form the fioor 
of the fore gut. On either side of the gut the anterior extremities of 
the ecelomic cavities (c.) are present. 
The next section described is represented in Fig. 44. The section 
passes through the optic vesicles (0. v.) which at this time are hollow 
and in wide communication with the fore brain. On either side there 
are slight depressions of the superficial epiblast which are probably arti- 
facts due to killing reagents. The loosely scattered cells of the mesoblast 
(mes.) form two large masses which extend laterally from the region 
where the floor of the fore brain rests directly upon the gut hypoblast. 
Just beyond the lateral boundary of the fore gut the mass separates into 
two layers, an outer somatic which is closely united with the ectoblast 
and an inner splanchnic which lies close to the gut hypoblast. Between 
these two layers are the coelomic cavities (c.). The gut (f. g.) is here 
widely expanded and, on either side, are seen the hypoblastic cells as they 
pass over into the anlagen of the adhesive organs. 
A section through the region where the mid brain passes over into the 
medulla is show in Fig. 45. The pharyngeal portion of the gut is here 
widely open and slight evaginations indicate the first appearance of the 
visceral clefts. Just external to these are mesoblastic masses (v. d.) 
which are the beginning of the visceral arches. The mesoblast extends 
down around the brain until it comes in immediate contact with the 
notochord. The walls of the ccelomic cavities (c.) are separated widely 
and are lined by a single layer of cells. 
Passing still further back we have selected a section (Fig. 46) through 
the region of the auditory vesicles. The cavity of the medulla is here 
widely open. Its lateral walls are made up of elongated epithelial cells 
arranged in one or two layers. Its roof, however, is very thin, so that 
when viewed from the surface it is very transparent. On either side are 
the auditory vesicles (a. v.) which have formed from thickenings of the 
deeper ectoblast. Above these the two layers of the superficial ectoblast 
are continuous. The vesicles which were earlier solid now show very small 
lumina. Just external to the vesicle on the right side there is a divertic- 
ulum of the gut, the pharyngeal portion of the third cleft, while just 
outside this, a thickening in the mesoblast is the third arch. The cells 
of the hypoblast forming the dorsal wall of the gut are flattened, but in 
the region of the clefts they become cuboidal, which character they retain 
until they pass over into the yolk cells. The ventral wall of the gut is 
