OF THE GERMINAL LAYERS IN GASTEOPADA. Öi> 



especially concerning the fate of the cells. Kevertheless the 

 alteration of the body-form in this species seems to be not so 

 pronounced as in those I have studied. Holmes seems to think 

 that the ultimate cause of such a transformation lies in the dif- 

 ferent behaviour of the posterior cells of the third ectomere 

 generation. Thus he goes on to say:- "we may view the earlier 

 division of the cell oa and Sb as the first foreshadowing of 

 bilateral cleavage." • By this expression it is quite evident that 

 there is no special cell formation as in the cases described in the 

 present paper. Such a gradual modification as is elucidated by 

 the author is of a wide occurrence in the Molluscan egg and the 

 change of symmetry by the cell formation near the posterior end 

 of the body is demonstrated at present only in the species studied 

 by myself. Nevertheless I believe that the same fact will here- 

 after be confirmed in other forms. 



It may not be useless to summarize here the main points 

 brought out in the present investigation. 



1. Throughout the whole process of cleavage it is observa- 

 ble, that there is no fixed regularity in the course such as is 

 expressed in the so-called law of alternation of spirals as stated 

 by Wilson, Kofoid et al. Even the corresponding daughter- cells 

 from the same blastomeres or ectomere-cells are propagated dif- 

 ferently in different forms. The first generation of ectomere-cells 

 is produced in S'qjhonaria and in Aplysia in spirals of opposite 

 directions and even in the same species the cleavage sometimes 

 takes place consecutively in the same direction, and not alternately 

 to rio'ht and to left as has been observed in other forms. 



