TO THE EMBRYOLOGY OF AMPHIBIA. 45 



As above stated the egg-rotation is a fact which has already 

 been noticed by all investigators in the growing eggs of Amphibia. 

 But the degrees passed by the rotation as well as the period at 

 which the rotation takes place, are different according to different 

 writers. Thus according to Pflüger ('83), Koux ('88«), Morgan 

 ('93Ô, '97) et al. the egg has to rotate, about 170° or more, 

 after the complete closure of blastopore. O. Schulze ÇSSb) 

 maintains that the egg rotation begins from the beginning of the 

 gastrulation process, so that it counterbalances, from the first, the 

 downward growth of the dorsal blastopore lip over the yolk 

 hemisphere. 



Meanwhile the fact that the egg-rotation takes place by the 

 change in position of the center of gravity of the egg during 

 the gastrulation, has already been noticed by all my predecessors, 

 and is now generally accepted. 



I will now go over categorically some of the other points 

 brought out in the study of the sections : — 



1). The growth in size of the segmentation cavity from the 

 condition seen in Fig. 51 to that of Fig. 53 is no doubt due to 

 the thinning out of the roof which, as mentioned before, is three 

 or four cells thick at first but becomes later strictly two-cell 

 layered. This thinning appears to me to be brought about by the 

 downward shifting or migration toward the equator of the cells 

 composing the roof which no doubt also increase by division. 



2). It seems probable to me that the equatorial zone is 

 produced by the accummulation of these downward migrating cells 

 rather than by the multiplication of the cells which were here 

 from the first. This view is strengthened, it seems to me, by 

 what is seen in Figs. 63 and 64 which are respectively the 

 magnified sagittal sections of the zonal mass at the blastoporic, 



