24 ART. 1. T. FUJITA : ON THE FOKMATTOX 



the posterior portion of the l)Ocly so as to lie ahnost on tlie same 

 level with its complementarv cell re on the opposite side. 

 By comparing Figs, ^"^ß and ö?, the optical liorizontal sections of 

 two different stages, we are aide to perceive this change in the 

 position of the left bilateral cell. Idms it seems that the final 

 position of the bilateral cells in the two species under considera- 

 tion coincides in almost every respect. I am not in position to 

 contradict the views expressed by previous authors, concerning 

 the transition from the radial to the Inlateral symmetry ; l)ut at 

 least in the species that T h.ave studied I am convinced that the 

 Itilateral cells are the main, if not the sole, cause of the trans- 

 formation of the l)ody symmetry. I should like to make a 

 reniîirk here on the same stage in A. limacina Blochmanx 

 makes no allusion to the phenomenon mentioned above. But a 

 close inspection of his figures shows us the occurrence of the same 

 events in that species. His Fig. 13 has an intimate relation to 

 the stage of the seventh ectomere generation, although it is 

 introduced to show an earlier sta2;e. There it is seen that c-, has 

 given rise to a small cell. This does not seem to form a quad- 

 rant of a quartet, lait corresponds in its general character to 

 the left bilateral cell. Again in his Figs. 14 and 15 the cell 

 r.y as well as the first entomere-cells are seen to occupy the same 

 position with those noted above by me. Although it is to 

 be gathered from the author's interpretation that the change of the 

 body symmetry occurs as early as the stage of about 24 cells, 

 yet his ligures incline me to believe that this phenomenon takes 

 place nuu'h latei-, just as in the case of the present species. 



Innnediately after the formation of the bilateral cells or 

 sometimes simultaneously with it there takes place the cleavage 

 of the iiiesomere M. A spindle appears lying in the transverse 



