MULTIPLICATION BY FISSION 1 49 



animal in detail, or even to indicate accurately the part which 

 the individual cells take in the process of budding. 



The question of the origin of the supplementary determinants 

 which we have assumed to exist in the cells of the epidermis, 

 does not stand in the way of this explanation of the regenerative 

 process ; for, as already stated, a similar course is in the main 

 followed both in embryogeny and regeneration. In both pro- 

 cesses the primary mesoderm arises from the primary ectoderm. 

 The definitive ectoderm cells have therefore an opportunity dur- 

 ing their embryonic development of taking over certain primary 

 mesoderm determinants as accessory idioplasm from the primary 

 ectoderm cells : these can then become separated into several 

 groups during the multiplication of the ectoderm cells, so that 

 the epidermic cells around the circumference of the body are 

 provided with an accessory idioplasm consisting of various meso- 

 derm determinants. 



It must, however, also be borne in mind that the growth in 

 length of the worm only takes place at the posterior end of the 

 body, just as occurs in the regeneration which prepares the way 

 for division. In both cases a new body-segment is formed be- 

 tween the last segment and the last but one, in which process 

 the epithelium of the intestine alone arises from the endoderm, 

 the integument and all the mesodermic structures being formed 

 from the ectoderm. Thus the accessory determinants which we 

 have assumed to exist in the epidermic cells, and which render 

 the subsequent regeneration possible, are not derived from the 

 embryo directly, but from the zone of growth in the tail-end, 

 into which again they have passed during embryogeny. 



3. The Process of Fission in the MiCROSTOMiDiE 



It is not, then, in the nature of every ectoderm cell to give rise 

 to all the possible kinds of cells and organs with the exception 

 of the epithelium of the alimentary canal : each one must be 

 specially equipped for the purpose. This is proved by the fact 

 that the ectoderm by no means always performs this function in 

 animals which multiply by fission : even in some worms this is 

 not the case. 



According to von Wagner's * excellent researches, the cells 

 of the epidermis in a certain flat-worm, Microstoma lineare, take 



* Loc. cit. 



