CLEAVAGE IN THE EGG OF CEREBRATULUS. 17 



protoplasmic differentiation and also that the cleavage plane of 

 certain cells may be transmitted to their progeny. It is extremely 

 difficult to assume such predetermined cleavage planes in the egg 

 of Cerehralulus as already mentioned and still more so in that of 

 Crepidula, in which vivid protoplasmic movements can be seen. 

 Tn understanding the factors controlling the position of the 

 spindle, one should not overlook another important one emphasized 

 especially by Ltllte ('99, '01), viz., adaptation in cleavage. 

 Though the cleavage pattern may be artificially altered without 

 causing any morphogenetic disturbances, yet it cannot be doubted 

 that each division under normal condition has prospective signifi- 

 cance. Moreover we must take into consideration that modifica- 

 tions in cleavage have taken place during the vast length of 

 phylogeny, rendering a mechanical explanation of the present mode 

 almost impossible. At the outset some of the processes might 

 have had a mechanical significance, but now they are incorporated 

 into what we might call inherent or " active physiological " (zur 

 Strassen) property of the egg. We may find an analogous case 

 in the Bomhinaior tadpole, in which at first the perforation of 

 the operculum was in all probability due to the mechanical stimuli 

 of growing arms beneath, but at present takes place without them 

 (Braus '06). 



Zoological Institute, 



Imperial University, Tokyo, 

 July 3, 1909. 



