20 AÎIT. 0. — S. IKEDA : CONTRIBUTIONS 



the closure of the blastopore had further progressed and the 

 ventral lip had been brought further backward so that the entire 

 blastoporic area could now be seen in the posterior view of the 

 egg. The area of the segmentation cavity still kept its position 

 exactly opposite the blastoporic area. 



Further progress in these changes may be seen in Figs. 

 32-35 which give the left side view of the same egg at suc- 

 cessive stages of development sketched respectively at 12:50, 2:04, 

 3:30 and 4:50 p.m., on the same day. These figures show that 

 while the closure of the blastopore is going on, both the dorsal 

 and ventral lips of it are gradually taken upward toward the 

 equator by rotation. There is, however, an apparent difference 

 between the behavior of the dorsal, and of the ventral lips beyond 

 a certain point. When the dorsal lip in its upward return has 

 reached about the point where it made its first appearance, it 

 apparently ceases to rise any further, while the ventral lip continues 

 to do so (Figs. 33, 34 and 35). This is undoubtedly due to the 

 fact that the dorsal lip, partaking of the general growth of the 

 whole periphery of the blastoporic area toward its center, grows 

 downward toward the center of the area, and the rate of this 

 downward growth must" be exactly the same as that of its upward 

 progress by the rotation of the whole egg, so that it remains 

 apparently stationary. On the ventral lip, these two motions are 

 combined, and there is naturally a steady upward progress. In 

 earlier stages, the rate of the egg-rotation must be faster than 

 that of the growth of the blastopore lip, so that there is an up- 

 ward movement of the whole blastoporic area toward the equator. 

 Conditions like those just described might lead one to suppose 

 that the blastopore in certain later stages is closed only by the 

 rapid overgrowth of the ventral lip. 



