22 AKT. 1. T. FUJITA : ON THE FORMATION 



tliits liiglily specialized method of the formation and charaeteristic 

 disposition of the cells, such as has not yet been desciibed in 

 other species, I think it is better to denominate these as the 

 bilateral cells, thus distinguishing them from other ectomere- 

 cells. With the ])irth of the bilateral cells the spiral symmetry 

 is materially and abrui)tly disturbed ; and the configuration of the 

 egg is at length completely transformed into a bilateral synnnetry. 

 Thus it is seen that the change of the symmetry takes ])lace in 

 this species at a })eriod more definite than in any others tlius I'ar 

 known to us. 



Aplysia (''^.1.1, *^2.i.i> ^-i.i.u d'l.i.u Fig. 54-55, PL III) : — In 

 Aplysia the seventh quartet arises from one diH'erent in order, but 

 of strictly the same genealogical derivation as the corresponding 

 one of the foregoing species. Here it arises from the third 

 generation, and indeed first from its left posterior-cell iL^^ thence 

 following the left-handed spiral. This state of things will be 

 fairly understood when we compare Figs. 54 and bïS, illustrating 

 the sagittal, and the horizontal, optical sections respectively. In 

 these two figures the ectomere-cell d-i.i alone is represented. It 

 is noteworthy that while the spindles of the cells heretofore 

 described as well as of the other quadrants of this same (juartet 

 {a-,,i^ etc.) lie almost horizontally the spindle in this cell is 

 seen to take a somewhat vertical position. Consequently, when 

 the new daughter-cell (/'o.i.i is budded off, it lies more ventrally 

 than the other (juadrants «2.1.1, />lm.i, etc. The daughter-cells thus 

 g(;nerated have a great öimilarity in general outline to those of 

 either the rilUi, or the sixth generations. They generally border 

 the ventral side of the egg, lying almost uutler the cells of the 

 second generation. 



Concomittantly with this atage or a little earlier (Figs. 00 



