44 ART. 3. S. IKEDA : CONTRIBUTIONS 



angles to each other in Woodcuts III., and IV., indicate the actual 

 vertical and horizontal axes drawn in full lines in the plate 

 figures. The relative position and size of the segmentation cavity 

 and the archenteron are easily understood from this series. 



If now these two series (Figs. 51-61 and Woodcuts I-IV.) 

 are carefully studied, it will become clear that the position of an 

 egg at a given stage is entirely due to the position of its center 

 of gravity. Up to the stage of Fig. 54 or of Woodcut IL, the 

 mass and the segmentation cavity are uniformly disposed with 

 regard to the initial vertical axis of the egg, hence the center of 

 gravity lies in this axis, and on its lower pole the egg rests. 

 With the formation and gradual enlargement of the archenteric 

 cavity which goes on hand in hand with the reduction in size of 

 the segmentation cavity, (Figs. 66-61 and Woodcuts III-IV.), the 

 dorsal and posterior part of the egg gradually diminishes in weight 

 while, on the contrary, the anterior and ventral part becomes 

 heavier by degrees. The center of gravity is, naturally, shifted 

 forward, but as the archenteron keeps its symmetry in regard to 

 the sagittal plane, this shifting takes place along that same plane. 

 Hence such rotation of the egg as we have noticed in the study 

 of the external features. In feet, it would be impossible for an 

 egg constituted as given in Woodcut IV., to keep the unnatural 

 position given in that figure : if left to itself, it must fall forward 

 and assume the position given in Fig. 60. 



That the rotation is sometimes continued after the complete 

 closure of the blastopore, is due without doubt to the fact that 

 at the time of the latter event, the archenteric cavity has not 

 yet reached its maximum size. The archenteron, however, at its 

 highest development never extends below the original vertical axis 

 and is confined within the dorsal half. 



