Ill DIFFERENTIATION 73 



from the animal and from the vegetative hemisphere in respect 

 of their developmental capacities. 



In the Nemertines there is a similar difference between the 

 capacities of animal and vegetative egg-fragments, removed 

 prior to fertilization (Yatsu) ; the vegetative quarter of the egg 

 gives rise to a larva without sense-organ, the animal fragment 

 of about the same size, to a larva with defective gut and also 

 without the apical organ ; so that apparently the factor on 

 which the development of this organ depends is somewhere in 

 the centre of the egg, that is, not in its definitive position. Into 

 that position it moves after the first cleavage, since removal of 

 the animal regions of both blastomeres does not interfere with 

 its development. At an earlier stage still (prior to the break- 

 ing down .of the germinal vesicle) any nucleated piece of an 

 ovum can give rise to a normal larva, whatever be the direc- 

 tion of the cut by which it is removed ; that is, since the 

 nucleus is in the animal hemisphere, a meridional, an oblique 

 or an animal fragment. 



The development of egg-fragments of Sea-urchins is said to 

 be always complete, provided the fragment be not too small 

 (Driesch) ; but this is a matter which requires re-investigation. 

 In the Ctenophora the development of isolated blastomeres is 

 always partial, at least in respect to the costae, J, , f, -Jf blasto- 

 meres giving rise to larvae with respectively 2, 1,3,5 costae, and 

 so on. The stomodaeum, however, of ^ and J larvae is complete ; 

 and a f larva has four and not merely three endodermal canals. 

 In the Mollusca as a rule a cell when isolated gives rise to no 

 more than it would have done had it remained in connexion 

 with its fellows, except that the cell-mass produced from it 

 may form a closed vesicle (for instance, isolated 1 a 1 d cells 

 of Patella (E. B. Wilson)), but in those peculiar cases (Ilyanassa, 

 Dentalium), where there is a polar lobe the cell in the two- 

 celled stage or four-celled stage which possesses it, i.e. either 

 CD or Z), can produce a whole larva. Removal of the polar 

 lobe from the whole egg involves absence of mesoderm in 

 Ilyanassa, absence of the post-trochal region, and of the apical 

 sense-organ in Dentalium. Removal of the lobe from the CD 

 cells involves in the latter genus the same consequences, but 



