THE EARLY DEVELOPMENT OF AMIA, 437 
fig. 15, and Pl. 32, fig. 34). Its substance is insunken in the 
egg. 
The mode of establishment of the early embryo’s outward 
form may be followed in PI. 30, figs. 16—20. In the first of these 
figures (fig. 16) the embryo surrounds about 185° of the egg’s 
circumference, and the blastopore has disappeared; a whitened 
line on the egg’s surface represents the neural axis, its en- 
larged terminal the brain tract ; a slight darkening in the axial 
line is due to a shallow trench-like insinking of the neural plate. 
The embryo is not as yet noticeable above the surface curva- 
ture of the egg. In PI. 30, figs. 17 and 18, the head and tail 
regions of a slightly older stage, the following changes have 
taken place:—The embryo, a rod-like surface thickening, 
surrounds about 195° of the egg’s circumference; its head, 
prominent and enlarged, rises slightly above the surface curva- 
ture of the egg; its trunk tapers hindward, ridge-like in form, 
closely apposed to and slightly insunken in the egg mass; two 
mesoblastic somites have appeared. In the stage of Pl. 30, 
figs. 19 and 20, six somites are present, and the embryo has 
enclosed about 220° of the egg’s circumference; the neural 
axis, now more sharply marked, rises slightly above the egg; 
the tail in its growth is now separating from the egg surface ; 
the brain regions are defined, optic vesicles are appearing ; and 
the head region, growing slightly forward, has in front the 
beginnings of the stomodzum. 
In the features of early outward growth Amia again presents 
marked transitional characters between Ganoids and Teleosts. 
Like the latter, its form growth is early concentrated in the 
sagittal plane, and the embryo deeply sunken in the egg mass. 
It later resembles the Ganoids in its uplifting from the yolk 
and in the details of organogeny. <A few further notes re- 
garding the origin and growth of its structures are given in 
the following section. 
G. The Origin and Early Differentiation of the 
Germ Layers.—The outer germ layer has been already 
noted (p. 431) in the early gastrula as a compact cell mass of 
comparatively uniform thickness. Its subsequent growth has 
