36 ART. 3. S. IKEDA I CONTRIBUTIONS 



calculated to bring out clearly the equal growth of the whole 

 periphery of the blastopore lips towards the yolk pole. 



4). The point at which the blastopore finally closes is in 

 reality always what is the lowest point of the yolk hemisphere 

 (or the yolk pole in s. sir.) of the unsegmented egg. The distance 

 traversed by the dorsal lip from its first appearance to the time 

 of closure is therefore about 70°-80° in Rhacophorus, and 68°-70° 

 or less in Bufo and Rana ; that by the ventral lip is about 

 20°-40° in all the species ; and that by the lateral lip ranges 

 between these two extremes. This estimation agrees well with those 

 given by Assheton (60°-70°), and Kopsch (circa 75°-80°). 



5). When the dorsal, ventral, and lateral lips of the blasto- 

 pore have grown down to a level which is 60°-75° below the 

 equator, the whole egg begins to rotate slowly on its transverse 

 horizontal axis, the resting point of the egg gradually shifting 

 toward what becomes the ventral face of the future embryo, and 

 the dorsal lip apparently returning toward its starting point. 

 This greatly obscures the actual mode of the blastopore closure 

 (or the equal growth of the lips over the yolk hemisphere). 



6). This rotation of the egg within the vitelline membrane 

 is very slow and may cease, sometimes before, and sometimes after 

 the process of the blastopore closure is completed. 



7). The dorsal lip of the blastopore in normally growing 

 eggs, therefore, never goes ventrad beyond the middle point of 

 the blastoporic area (the yolk-pole). 



8). In Rhacophorus, the correlation in size between the 

 upper translucent area of the segmentation cavity and the lower 

 blastoporic area becomes specially obvious in eggs fixed on the 

 mirror. They not only diminish in size together, but also main- 

 tain their position opposite each other, even after the rotation 



