48 AKT. 3. S. IKEDA I CONTRIBUTIONS 



is caused by the gradual growth of the archenteron which pushes 

 the yolk-mass ventrad and thus causes the segmentation cavity to 

 become obliterated in the ventral half of the egg. But according 

 to my own observations, the segmentation cavity becomes smaller 

 not only by the gradual growth of the archenteric cavity but also 

 by the rising upward of the yolk-mass which takes place along 

 the inner face of the epiblast, roofing over the segmentation-cavity 

 as the blastopore lips grow downward. In other w r ords, the cavity 

 becomes reduced eentripetally, in situ. Thus as shown in Figs. 

 57, 58, and Woodcut IV., the segmentation cavity and the blasto- 

 pore area both of which have now become very small are found, 

 as before, at, or close to the former vertical axis of the egg, 

 without changing their original positions relative to each other. 

 So far as my experience goes, I have never yet met with a 

 case in which the final point of the disappearance of the seg- 

 mentation cavity was below the original vertical axis of the egg 

 (the dotted line in Figs. 57, 58, and 60), although I am not in 

 a position to assert that such cases may not occur as individual 

 variations. Also, according to my views, the atrophy of the 

 segmentation cavity has not so much to do with the rotation 

 of the egg as the growth of the archenteric cavity, since in the 

 former process the yolk cells rise en masse so to speak, and do 

 not greatly change the position of the centre of gravity. 



As a case of individual variation, there is that of Fig. 57 in 

 which the segmentation cavity communicates freely with the archen- 

 teric cavity, the barrier separating the two cavities having broken 

 down. Such a case is often met with. In fact, seeing that the yolk 

 mass in the living egg is not solid but must be very mobile, being- 

 composed of loosely associated yolk cells, it would be strange if such 

 and other slight differences were not found in various individuals. 



