446 



TOKlü KADURAKI : 



5. Reproduction by Fission. 



Planaria gonoccpJiala Dugès, as observed by me in Japan, seems 

 to reproduce to a large extent by fission. I have never yet met with 

 individuals which were doubtlessly right in the process of dividing, but 

 those bearing evident marks of foregone fission are rather common. To 

 judge from these, it seems that this always takes place at a point 

 immediately behind the pharynx or a short distance farther back", much 

 as is known from Pi. maculata Curtis. Instances of division in front of 

 the pharynx, known to obtain in PI. albissima Sekera, was never 

 observed in the species in question. The severed end of the two pieces 

 produced by division presents for some time a transverse and nearly 

 straight edge, exposing the parenchyma in a thin white line. For some 

 time after the healing of the wound, there is observed at the body-end 

 concerned a colourless area of regeneration, which area, bounded off 

 from the old parts by a zone of concentrated pigmentation, is at first 

 of a crescentic, and then takes a more or less triangular, shape. In the 

 tail piece that area develops into the part of the new head anterior to the 

 auricles, while in the head piece it merely forms the new tail-end. In 

 the former case the regenerated area increases considerably in extent, 

 and the eyes appear as two minute dots close to its border against the 

 pigmented old body-parts. The points of the auricles begin to show 

 themselves somewhat later. 



PI. vivida and Polycelis ijimai Kab. also very frequently reproduce 

 by fission. In the former the division takes place quite similarly as is 

 known in PI. alpina Dana. First a furrow appears at the place where 

 division is to occur; it deepens, finally leading to separation of the 

 parts. There were not infrequently found cases in which the dividing 

 body-parts were still connected by a narrow band of tissue, which no 

 doubt will soon break off by the motion of crawling. 



In all the' species mentioned, reproduction by fission takes place 

 most actively during June, July and August, in which period it is 



