Ill DIFFERENTIATION 71 



plasm consists usually of a finely granular ectoplasm, and 

 a coarsely granular endoplasm. The first four divisions 

 meridional, meridional, equatorial, and meridional are all 

 perpendicular to the surface, and each blastomere has a share 

 of each of these two substances in the same proportions as 

 they exist in the whole egg. Experiment shows that when 

 separated from one another half-, quarter-, one-eighth-, and 

 even one-sixteenth-blastomeres will develop into whole 

 hydroids or medusae as the case may be. In Carmarina, how- 

 ever, there is in addition a jelly-plasm, a spherical hyaline 

 mass placed excentrically near the vegetative pole. This is 

 really a precociously secreted mesogloea and passes normally 

 into the exumbrella of the medusa. Isolated half- and quarter- 

 blastomeres, being produced by meridional divisions, naturally 

 each include a portion of this as well as of the other two 

 materials, and they can develop into whole medusae. But 

 the one-eighth-blastomeres, which must either contain none or 

 relatively too much of the jelly -plasm, can only develop par- 

 tially. In Aegineta, another medusa, there is also a jelly- 

 plasm in the egg, but centrally placed. The one~eighth-blasto- 

 meres are here totipotent. 



Amongst Vertebrates it is possible to obtain a whole larva 

 from each of the first two blastomeres in the Newt, at any 

 rate when the first furrow lies in the (invisible) plane of egg- 

 symmetry, and in the Frog we know that a double monster 

 can be produced by turning the egg upside down in the two- 

 celled stage, and so causing each blastomere, by rearranging 

 its contents, to acquire a new polarity of its own, In both 

 these cases, since the first furrow is meridional, each of the 

 blastomeres has a share of the various substances of the cyto- 

 plasm which are placed symmetrically around the axis of the 

 telolecithal egg. We should expect perhaps that after the 

 third, latitudinal, division the potentialities of animal and 

 vegetative blastomeres would differ. The evidence slender 

 though it is shows that this is not altogether so, for a 

 blastopore and archenteron may be developed not only from 

 the four vegetative cells when the others have been destroyed 

 (Morgan), but also from the animal cells alone (Samassa). 



