208 INTERNAL FACTORS IV. i 



separation, each develops into a Pilidium with one lappet but no 

 apical organ; it seems, therefore, that the material for this struc- 

 ture is at this stage placed in the bridge of plasma connecting 

 the two cells, and is fatally injured by the operation. 



Taken together these experiments make it, to say the least, 

 highly probable that there are in the Nemertine egg definite 

 substances connected necessarily, that is causally, with the forma- 

 tion of certain organs. 



These specific organ-forming substances may be said to be 

 preformed, but it is equally clear that they are not prelocalized, 

 but only reach their ultimate destination in the course of develop- 

 ment. 



LITERATURE 



E. B. WILSON. Experiments on cleavage and localization in the 

 Nemertine-egg, Arch. Ent. Mech. xvi, 1903. 



N. YATSU. Experiments on the development of egg-fragments in 

 Cerebratulus, Biol. Bull, vi, 1904. 



C. ZELENY. Experiments on the localization of developmental factors 

 in the Nemertine egg, Journ. Exp. Zool. i, 1904. 



8. CTENOPHORA. 



With the Ctenophora we come to a group of animals in which 

 the development of a total larva from a single blastomere is no 

 longer possible, though the missing parts may eventually be 

 regenerated. 



The egg consists of a central yolk-mass surrounded by a super- 

 ficial layer of granular protoplasm. After fertilization this 

 protoplasm passes wholly to one side the animal hemisphere and 

 the egg divides meridionally into equal parts. The protoplasm 

 then spreads itself over the outer surface of each cell, but returns 

 to the animal side prior to the second cleavage, which is again 

 meridional and at right angles to the first. Once more the 

 protoplasm is distributed over the whole outer surface of each 

 blastomere, only to be again concentrated before the third division, 

 an unequal one, but nearly meridional. There are now four large 

 cells lying together in a square and four smaller cells lying above 

 them in two groups of two each, one at each end of the plane of 



