MULTIPLICATION BY FISSION 147 



spending three daughter-individuals takes place by a construction 

 in the middle of each zone of regeneration. 



In Nais the zones of regeneration are always formed at the 

 boundary line of two segments : that is to say, they arise from 

 the contiguous margins of two segments, in the following way. 

 Cells of the epidermis first begin to multiply, and give rise to a 

 circular layer of small stratitied cells, which is thickest on the 

 ventral side. The cells have at first no definite histological 

 character. At the same time an increase in the length of the 

 internal organs takes place : this is rendered necessary by the 

 growing zone penetrating between the segments from which it 

 arises and thus forcing them apart. The alimentary canal, how- 

 ever, is the only internal organ which becomes regenerated from 

 its own cells : all the other new formations, including the ventral 

 nerve cord, muscles, blood-vessels, 'liver '-cells, and excretory- 

 organs, are developed from the ring of proliferating epidermic 

 cells. 



As Semper has pointed out, the process of the reconstruction 

 of the anterior and posterior ends which prepares the way for 

 fission, may in a sense be compared with the embryonic develop- 

 ment of the animal subsequent to the gastrula stage, in which 

 the two primary germinal layers are already distinct. 



In these regenerative processes two layers of formative cells 

 are likewise produced, owing to the proliferation of the ectoderm 

 cells on the one hand, and those of th^gipPRntary canal on the 

 other ; the epithelial lining of the latter only is formed from the 

 internal layer, the outer layer g^ing rise to all the other organs, 

 including the mesodermic structures as well as those which 

 belong to the ectodermic part of the integument. In fact the 

 resemblance between the processes which take place in embry- 

 ogeny and in regeneration is so close, that in both cases the 

 mesoderm becomes split off from the mass of formative ectoderm 

 cells in the form of two longitudinal bands, from which the 

 blood-vessels, muscles, &c., are then differentiated. 



In order to explain these processes theoretically from our 

 point of view, we must suppose that those cells of the epidermis 

 from which the formative cells arise possess an ' accessory idio- 

 plasm, containing the determinants of those organs which are 

 formed from them in regeneration in addition to their own 

 specific idioplasm. The rate of division of each of these cells. 

 as well as the manner in which the groups of determinants con- 



