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numerous intercellular depressions, angular and irregular, with pig- 
mented walls, developed. 
In the egg selected for observation the dorsal lip had appeared, 
but was still a short, slightly curved line, at 6.30 P.M. Diameter of 
compressed egg was 105 micrometer divisions. A cell boundary in 
the yolk, 5 div. from the dorsal lip, was selected at 6.30 P.M. Gra- 
dual approximation of lip and cell boundary took place, lip reaching 
cell at 8.45 P.M. The conclusion drawn is that the dorsal lip moves 
5 div. towards lower pole 6.30—8.45 P.M. 
During this time, 6.30—8.45 P.M., the rolling over of ectoderm 
cells round the dorsal lip was observed. The accumulation of pigment 
round the nucleus in these cells made a dark spot. Spots of this sort 
4—5 div. from the lip were easily observed gradually to approach, 
and finally to disappear, round the lip. 
At 9.10 P.M. the ventral lip was outlined as a distinct, though 
still rough and jagged line; dorso-ventral diameter of the blastopore 
measuring 70 div. A cell boundary in the yolk 7 div. from the ventral 
lip was selected. The lip gradually approached the cell boundary; and 
at 11 P.M. was only 2 div. distant. The conclusion drawn is that from 
9.10 to 11 P.M., the ventral lip moves 5 div. over the yolk. By 11 P.M., 
the dorso-ventral diameter of the blastopore is only 60 div.; egg dia- 
meter still 105 div. The inference is that the diminution of 10 div. 
in the size of the blastopore, between 9.10 and 11 P.M., is in part 
due to overgrowth of the ventral lip (5 div.), and in part to the con- 
tinued overgrowth of the dorsal lip (5 div.). The opposite parts of 
the lip advance at equal rates. Meantime (9.10—11 P.M.) particular 
ectoderm cells, 4--5 div. from the lip, gradually approached and dis- 
appeared round the lip. On the following day, neural folds were well 
developed. 
In some of the other experiments, the progress of the ventral lip 
over the yolk was followed somewhat farther than in the just de- 
scribed case. In one such, the diameter of the blastopore when the 
ventral lip is first outlined as an edge (at 11.15 P.M.) is 65 div., 
while the egg diameter is 105 div. From 11.15 P.M. to 1.45 A.M., 
the progress of the ventral lip over the yolk was watched for 8 div. 
At the end of this time, the blastopore diameter was 50 div. The 
diminution of 15 div. in the size of the blastopore is again due in 
part to the movement of the ventral, and in part to the movement 
of the dorsal lip. The two lips progress at about equal rates. 
Sections indicate that during the process of gastrulation, the yolk 
