IV. i INITIAL STRUCTURE OF THE GERM 241 



is capable of segmenting 1 when deprived of its vegetative hemi- 

 sphere, but the larva to which it gives rise has no combs and no 

 sense-organ. The egg of the Ascidian Cynthia exhibits in its 

 cytoplasm various substances, easily distinguishable by their 

 colour ; in segmentation these are distributed to the various blasto- 

 meres in a perfectly definite way. Remove one or more of these 

 substances and the embryo will lack the organ or organs normally 

 produced from them. Again, a Nemertine egg from which, at 

 a certain stage, the vegetative portion has been cut away, gives 

 rise to a Pilidium without lappets and without a sense-organ. 



In these cases, then, at least, the existence of specific organ- 

 forming stuffs in the cytoplasm of the egg has been proved ; 

 the removal of the stuffs inevitably entails the absence of the 

 corresponding organ later on. But though it may therefore be 

 justly said that such stuffs are preformed, it is not invariably 

 true that they are prelocalized, present, that is to say, ab initio in 

 what will be their eventual situation. For example, the stuff 

 which determines the formation of an apical organ in the larva 

 of Dental I Jim is originally in the polar lobe of the vegetative hemi- 

 sphere : but between the first and the second divisions it migrates 

 into the animal portion of the egg ; and the same may be said of the 

 formation of the sense-organ in Cerebratulu* and the Ctenophora. 



It seems highly probable, though it cannot be said to have 

 been demonstrated, that such specific organ-forming substances 

 are of universal occurrence. 



The behaviour of the isolated blastomeres of the eggs of 

 different forms, as well as of the blastomeres of the same form 

 at different stages, is markedly different. The ^ or \ blastomere 

 of an Ascidian egg can never produce more than \ or \ larva ; 

 the isolated blastomeres of Patella produce just so much, dividing 

 just so many times, as they would have produced had they 

 remained in connexion with their fellows; a radial group of 

 blastomeres (one or more | blastomeres, or two or four or 

 more ^ blastomeres) of a Ctenophore egg gives rise to a larva 

 with one, two or more costae and meridional canals, as the 

 case may be. On the other hand, the isolated ^ or i blastomeres 

 of a Nemertine ovum will produce a complete larva, while, after 

 the next division, the totipotentiality is lost; for an isolated 



JENKl.NSON 



