140 Gastrulation and Embryo Formation in Amia Calva 
a slighter thickening of the margin gives rise to a condition which reminds 
one of the germ-ring of the teleost. 
An equatorial section, taken just above the equator of an ege in the 
same stage, is shown in Fig. 30. The ectoblast in the embryonic region is 
much thicker than elsewhere, and from this region of greatest thickness 
it shades off gradually on either side, showing that at this time there are 
no well-defined lateral boundaries of the embryonic anlage. Just be- 
neath the median portion of the embryonic anlage there is a compact ar- 
rangement of the mes-entoblastic cells which represents the beginning of 
the notochord (ch). 
Egg Fifty-five Hours After Fertilization. Blastodisc Covers About 
240°. Embryo Extends Over 110°-120°.—The embryo now presents a 
profile (Fig. 6) which may be spoken of as somewhat triangular. Its 
anterior portion fades out in the region of the upper pole of the ege. Its 
posterior portion, however, is more sharply defined owing to its being 
deeply infolded at the blastoporic margin. In many embryos of this stage, 
there is present a median thickening in the blastoporic margin which 
may doubtless be considered as the homologue of the caudal knob of the 
teleosts. The lower portion of an egg in about the same stage is shown in 
Fig. 7. It will be noticed that the margin of the blastodise is not only 
deeply infolded along the base of the embryo, but also slightly infolded 
on the opposite side of the egg. A comparison of Figs. 6 and 7 with 
Fig. 5 shows that the surface cleavage of the yolk is very slow. 
A meridional section of an egg in this stage is shown in Fig. 31. The 
embryonic anlage here shows as a thickening of the ectoblast. The area 
of maximal thickness (i) near the upper pole of the egg represents the 
anlage of the head. In this region the superficial ectoblast shows no 
changes, the thickening being due to the proliferation of the deeper ecto- 
blast which is now twelve to fourteen layers thick as compared with six 
to eight in the preceding stage. The deeper layers decrease in number 
throughout the anterior trunk region and again increase at the blastoporic 
margin. In front of the anlage of the head (h), the deep ectoblast becomes 
thinner until, in the region of the equator, it is but a single layer 
thick; beyond this region it again thickens and at the blastoporic margin 
is three or four layers deep. 
The segmentation cavity, which in the preceding stage extended over 
the greater portion of the upper hemisphere, is no longer present above 
the level of the equator. The layer of ectoblastic cells forming its roof 
is still sharply diffrentiated from the other layers of the ectoblast. 
The gastral cavity (g. c.) has extended cephalad to the level of the 
posterior third of the embryo. Behind the dorsal lip of the blastopore, it 
