THE SPIRAL TYPE OF CLEAVAGE. 241 



A very interesting result obtained by Crampton is the failure 

 of those eggs in which the yolk lobe had been removed to form 

 a cell corresponding to the mesoblast in structure, although a 

 cell which is packed with yolk spherules, like the other entoder- 

 mal cells, is formed. According to Crampton the yolk lobe 

 appears to consist almost wholly, or wholly, of yolk spherules ; 

 and in any case it cannot contain the bulk of protoplasm repre- 

 sented by the mesoblast, so that it is apparently not the loss of 

 mesoblast material, but some other effect of the mutilation which 

 results in the defect. The result of this experiment indicates, 

 it seems to me, a considerable amount of rearrangement of the 

 egg substance. 



But perhaps the most interesting result of all these experi- 

 ments is the extreme sensitiveness of the egg to injury, as in- 

 dicated by the very small proportion of mutilated eggs which 

 continued to develop. This fact points most unmistakably to 

 the existence of a very high degree of interdependence among 

 the blastomeres. In most cases the portion of the egg observed, 

 although itself apparently uninjured, is so crippled by the loss 

 of the part removed that death ensues. Even in those cases 

 where development continued for a time the embryo never suc- 

 ceeded in completing it, but sooner or later succumbed. 



Wilson succeeded in modifying the cleavage of Nereis by 

 means of pressure, so that the micromeres were not formed till 

 the fourth division, and then eight of them appeared above 

 eight more, larger, yolk-laden macromeres. These eggs devel- 

 oped into trochophores with double the usual number of ento- 

 dermal blastomeres. Here again the insignificance of the cell 

 as compared with the organism is apparent. 



I have not had access to Fujita's paper upon Aplysia, but in 

 a brief abstract of it, it is stated that he found that isolated 

 blastomeres were able to restore the lost portions and to reas- 

 sume the normal shape. Whether the rearrangement occurs 

 directly or in later stages is not stated. In any case the effect 

 of the mutilation upon the remaining portions is evident. If 

 complete rearrangement does take place, it may be because the 

 egg of Aplysia is more hardy than that of Ilyanassa. 



Turning now to the normal cleavage, we find there a number 



