Albert C. Eycleshymer and James Meredith Wilson 139 
face view (Fig. 5). It first appears as a ight area with ill-defined out- 
lines extending over some 80° towards the upper pole, where it shades 
off imperceptibly into the remainder of the blastodise. That portion 
which is invaginated to form the dorsal lip of the blastopore stands out 
more prominently from the yolk than elsewhere. The line of invagina- 
tion now extends some 60° along the margin of the blastodise and ap- 
pears as a crescentic fissure. 
A meridional section of an egg in a stage closely corresponding to that 
described above is represented in Fig. 27. Although this egg is but 
slightly older than the one shown in Fig. 26, some interesting changes 
have occurred. In the preceding stage the deeper ectoblast was seven or 
eight layers of cells thick; now it is only two or three. The embryonic 
margin of the blastodise has likewise undergone a reduction, while the 
opposite side of the blastodise is reduced to one-half the number of layers 
present in the preceding stage. In addition to these changes, the stratum 
of cells which we have designated as mes-entoblast extends well up toward 
the upper pole of the egg and it is probably through the extension of this 
layer that the surface views show faintly the anlage of the embryo. 
In most eggs in this and subsequent stages, there are relatively few 
entoblastic cells as compared with the earlier stages. The space which 
in most of the earlier stages was filled with small cells is now filled with 
large yolk masses with a few smaller cells scattered among them. It is 
possible that many of the entoblastic cells have found their way into the 
rapidly extending blastodisc. 
Fig. 28 represents a section of the blastoporic end of the embryo under 
much higher magnification. The section is taken from another egg in 
about the same stage of development as that shown in Fig. 27. It will 
be here noted that the gastral cavity is lined above by a single layer of 
cells which strikingly resemble those of the superficial ectoblast in size, 
granular contents and staining capacity. The same can be said of the cells 
forming the ventral wall. The cells of this wall rest in this section 
upon the large yolk masses whose margins are regular and clearly de- 
limited. A peculiar feature which was noticed in the preceding stage, 
but is here more clearly shown, is the striking differentiation of the inner- 
most layer of the deep ectoblast. These cells stain more deeply than the 
remaining cells of this stratum. 
Fig. 29 represents a meridional section of an egg in a stage somewhat 
later than that last described, but earlier than the stage shown in Fig. 6. 
The blastodise has undergone continual thinning at the upper pole until 
at present it is but two or three layers of cells thick. At the blastoporic 
margin the blastodise is thickened, while on the opposite side of the egg 
