THE EARLY DEVELOPMENT OF AMIA. ASL 
latter is figured the stage of the closing blastopore ; the embryo 
is here faintly outlined as a white flattened cell mass; at its 
hindmost region the blastopore, as a funnel-shaped pit, is en- 
closed by its thickened and constricting margin. 
It is, however, from the study of gastrulz in sagittal sections 
that the most interesting comparisons may be made with the 
conditions in gar-pike, sturgeon, and Teleosts. Some of these 
have been figured in Pl. 32, figs. 31 (=PIl. 30, fig. 11); 32 
(=Pl. 30, fig. 12); 33 (=PI. 30, fig. 14) ; 34 (slightly earlier 
than Pl. 30, fig. 15). But before comparisons may be estab- 
lished, the advancing changes should briefly be reviewed. 
The early gastrula, Pl. 32, fig. 31, it is to be noted, occupies 
approximately the area of the egg’s surface as the blastula 
of Pl. 31, fig. 830; it has, however, the following advancing 
characters :—the loose cells of the blastoderm, now of minute 
size, have flattened into a compact cell mass, ectoblast (0) 
still presenting a well-marked surface layer (0’) ; the segmenta- 
tion cavity (sc.) has accordingly become greatly depressed, and 
is now fissure-like ; below it are several tiers of loosely com- 
pacted cells, which, by regular transition, appear to take their 
origin in large, vaguely defined yolk cells (yc.) arising from 
merocytes (m’); the rim of the blastoderm at di. is the region 
where the blastoderm is early separated from the yolk; there 
is here, however, no cavity apparent separating the dorsal lip 
of the blastopore (d/.) from the yolk (y.); both are closely 
apposed, and at the point * their elements can no longer be 
distinguished; the entoblast (7.), arising from the undiffer- 
entiated tissue of the dorsal lip, is here composed of compact 
elements, but in the central region of the blastoderm becomes 
equivalent to the loose cellular layer already noted as con- 
tinuous with yolk-cells and merocytes; at the opposite point 
of the blastoderm’s margin, the blastopore’s ventral lip, no sepa- 
ration of the germ layers from the yolk has as yet occurred. 
In a following stage (Pl. 32, fig. 32) the blastoderm is shown 
enclosing about 285° of the egg’s vertical circumference. The 
ectoblast has greatly thinned out, is thickest at its perimeter 
aud at its dorsal lip; the segmentation cavity is fissure-like ; 
