Albert C. Eycleshymer and James Meredith Wilson 141 
extends around on either side of and behind the large yolk plug where it 
is continuous with that part of the cavity which is everywhere lined ex- 
ternally by a sharply differentiated layer of hypoblast. At the blastoporic 
margin, this layer of hypoblastic cells changes in character from the small 
elongated cells with deeply staining granules to larger cuboidal cells and 
these in turn shade off into the smaller elongated cells of the superficial 
ectoblast. The floor of the anterior portion of the gastral cavity is made 
up of entoblastic cells which are heavily laden with large yolk granules. 
Toward the exterior these cells increase in size as they extend over the 
sides of the yolk plug until they finally become continuous with the great 
yolk masses (¥. m.) 
The layers of mes-entoblast (m. en.) not only extend much farther for- 
ward in the embryonic region but also become well differentiated in the 
extra-embryonic portion of the blastodisc. By tracing these layers in 
serial sections it is readily found that the anlage of the mesoblast is 
peristomal. 
Embryo Siaty Hours After Fertilization. Blastodisc Covers About 
245°. Embryo Extends Over 130°.—The outline of the embryo as yet is 
indistinct in the anterior region, but fairly well defined posteriorly (Fig. 
8). The entire margin of the blastodisc is deeply infolded around the 
projecting yolk plug. In the posterior portion of the embryo, there is a 
shallow groove present. A comparison with other embryos in this same 
stage shows that this groove is variable, being sometimes more and some- 
times less pronounced. Sometimes it terminates posteriorly in a deep 
indentation in the margin of the blastopore much like the condition ob- 
served in Batrachus or Ameiurus; at other times there is a well-defined 
caudal knob. 
The sagittal section represented by Fig. 32 is from an egg in the same 
stage. The deep ectoblast in the head region is notably thickened, being 
now twelve to sixteen layers in dorso-ventral thickness. In the trunk 
region these layers are further reduced while at the blastopore they re- 
main practically unchangd. Anterior to the region of maximal thickness 
the deep ectoblast gradually thins until, as in the preceding stage, it is 
but one or two layers thick in the equatorial region; finally at the ventral 
lip there are four or five layers. 
The mes-entoblast has extended farther toward the upper pole, but to 
just what extent it is impossible to say since the cells are here indis- 
tinguishable, on the one hand, from those of the deep ectoblast, and on 
the other, from those of the yolk. At the blastoporic margin where 
the cells of the mes-entoblast and the deep ectoblast unite, they form a 
sharp angle. In this angle there now appears a peculiar group of cells 
