Albert C. Eycleshymer and James Meredith Wilson 13 
A study of meridional sections of many eggs in this stage shows that 
the blastodise takes on different forms. In most eggs it is distinctly 
erescentic, but in some it is lenticular. When it takes on the crescentic 
form, as shown in Fig. 21, there is often a very distinct segmentation 
cavity (s. c.) present. The roof of the cavity is here made up of five or 
six layers of cells. The cells of the blastodise contain finer granules than 
those contained in the large yolk segments. At the margin of the blasto- 
disc, the cells pass over into those of the yolk by such imperceptible grada- 
tions that no sharp line of demarcation can be seen. The outermost layer 
of the blastodise may be designated as the superficial layer of the ecto- 
blast (s. ec.) and as stated by Whitman and Eycleshymer it early ap- 
pears quite unlike the deeper ectoblastic layers (d. ec.), in that it pos- 
sesses granules which stain more intensely than those in the other layers. 
The upper ends of the yolk masses (y. m.) are, in the egg shown, 
smooth and only at the margin are the cells being cut off. Other eggs, 
however, show that the large yolk masses at the center of the egg are ac- 
tively contributing to the blastodisc. In the section shown (Fig. 21) the 
yolk nuclei lie near the upper margin of the large masses and this upper 
portion is probably to be considered as homologous with the periblast of 
bony fishes. Not more than one-third of the cleavage grooves observed 
on the surface of the yolk have reached the center of the egg, leaving the 
yolk masses incompletely cleft and thus forming a great syneytium. 
Egg Twelve Hours After Fertilization. Blastodise Covers About 
110°.—The surface view (Fig. 2) shows that the rapid multiplication 
of the cells in the margin of the blastodisc has now given rise to greater 
uniformity in the size of all the cells of the blastodise. Beyond this 
feature, surface views show no points worthy of special mention. Meridi- 
onal sections (Fig. 22) show that the cells forming the superficial ecto- 
blast (s. ec.) are smaller and more elongated than in the preceding stage. 
The lower layers of cells of the blastodise are scattered through the upper 
portion of the segmentation cavity. The entoblastic cells, which have 
been cut off from the large yolk masses, are distinguished by their coarser 
granules and are also scattered through the segmentation cavity, many 
of them being found in its upper portion. Through these changes the 
segmentation cavity is more or less obscured. 
Egg Twenty Hours After Fertilization. Blastodise Covering About 
120°.—An egg of this stage (Fig. 3) shows a well-defined blastodise with 
a sharply delimited margin in which, under the magnification given, cell 
boundaries are no longer distinguishable. No features have been ob- 
served which enable us to recognize the embryonic anlage. The yolk 
shows little advance in cleavage beyond that described in the preceding 
