Albert C. Eycleshymer and James Meredith Wilson 137 
This thickening of the superficial layer marks the anlage of the forth- 
coming dorsal lip (d. J.) of the blastopore. The margin of the blasto- 
disc in the embryonic region has a more rounded contour than at the 
opposite margin. It is also thicker and possesses a greater number of 
cells with fine granules; the periblast, too, is more actively engaged in 
budding off derivatives in this region than at the opposite margin. ‘These 
several factors enable us at this time to orient the forthcoming embryo. 
The so-called periblastic nuclei are no longer confined to the upper 
margin of the large yolk masses, but are often widely scattered. These 
yolk masses sometimes contain several nuclei and the same is true of the 
scattered entoblastic cells. In other words, nuclear division here goes on 
far in advance of cytoplasmic division. 
Egg Fifty Hours After Fertilization. Blastodise Covers About 180°. 
—If the surface of the egg be carefully examined in a stage intermediate 
between Figs. 4 and 5, it will be found that just above the equator on the 
side of the blastodise which is least transparent there is a slight indenta- 
tion which indicates the beginning of the blastopore. As development 
progresses this indentation becomes a groove which extends in a latitu- 
dinal plane until it reaches the condition shown in Fig. 5, where it ex- 
tends around some 20° of the egg’s equatorial circumference. 
The meridional section represented in Fig. 24 is from an egg inter- 
mediate between those represented in Figs. 4 and 5. At this time, the 
blastodise has taken a more definite form owing to the greater compact- 
ness of its layers. In extent it covers very nearly one-half of the egg’s 
surface. It is noteworthy that at the time the blastopore appears the 
blastodise reaches its maximal thickness. 
In the particular egg described, the entoblastic cells are less densely 
aggregated than usual, with very large intercellular spaces, while the large 
yolk masses extend well up towards the lower layers of the blastodisc. In 
this respect we find considerable variation. In some eggs they are even 
less densely aggregated than shown in Fig. 24 so that a well-marked 
segmentation cavity (s. c.) is shown between the large yolk masses and 
the blastodisc. 
In the stage under consideration we have the first actual appearance 
of the invagination to form the archenteron. As to the factors which 
initiate this process, we are as much in the dark as ever. Without at- 
tempting to discuss the various theories, we may simply say that thus 
far there is an infolding of the external layer and that this infolding 
is not in the locality where the transition between large and small cells 
is most abrupt, but in a locality where the superficial cells are largest and 
of fairly uniform character. One would at first. glance think the in- 
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