146 



THE GERM-PLASM 



parts : this therefore neces- 

 sitates a regeneration of one 

 or of the other end of the 

 body, or even of both ends. 

 This process may begin to 

 take place either after (^Lum- 

 briadiis) or before the divis- 

 ion has taken place, and in 

 the latter case is more or 

 less complete before the fis- 

 sion begins. The actual 

 process of the formation of 

 the new organism is essen- 

 tially the same in both cases, 

 and important differences 

 only occur as regards the 

 various groups of animals. 



We are particularly well 

 acquainted with these pro- 

 cesses of regeneration — 

 which may either precede 

 or succeed fission — in the 

 case of various kinds of worms, and we will therefore first illus- 

 trate them in their main features by reference to these animals. 



Fig. 4. — Myrianida, a marine worm which 

 multiplies by fission (after Milne-Edwards, 

 from Hatschek's ' Lehrbuch der Zoolo- 

 gie ') . The letters a^g indicate the rel- 

 ative ages of the daughter-individuals 

 resulting from the division of the parent. 



2. The Process of Fission in the Naid^ 



The process of fission in these small fresh-water segmented 

 worms has been very accurately followed out by Semper. An 

 individual undergoes division into two, or usually into several, 

 daughter-individuals at the same time, the fission being regularly 

 preceded by a circular growth of cells taking place around the 

 circumference of the body at one or more definite regions, each 

 of such ring-like thickenings eventually giving rise to a new 

 head- and tail-end respectively. These rings of cells have 

 hitherto been spoken of as ^ zones of gemmation ; ' but it would 

 be better to call them ' zones of regeneration,' as they are not 

 concerned with budding in the true sense of the word. Two of 

 these rings are as a general rule formed in each animal, and 

 when the anterior and posterior ends of each of the resulting 

 three sections are fully developed, the separation into the corre- 



