340 



KEPBODUCTION. 



Fig. 357. 



Fig. 358. 



tion, is still more extraordinary ; it takes place only in polyps 

 and some infusoria. A cleft, or fis- 

 sion, at some part of the body takes 

 place, very slight at first, but con- 

 stantly increasing in depth, so as 

 to become a deep furrow, like that 

 observed in the yolk, at the begin- 

 ning of embryonic development ; at 

 the same time the contained organs 

 are divided and become double, and 

 thus two individuals are formed of 

 one, so similar to each other that it 

 is impossible to say which is the 

 parent and which the offspring. 

 The division takes place sometimes 

 vertically, as, for example, in Forti- 

 cella (fig. 357, c, d), and in some po- 

 lyps (fig. 358, a, d) ; and sometimes 

 transversely. In some infusoria, the 

 Parameciafor instance, this division 

 occurs as often as three or four 

 times in a day. 



513. In consequence of this 

 same faculty many animals are able 

 to reproduce various parts of their 

 bodies when accidentally lost. It is well known that crabs 

 and spiders, on losing a limb, acquire a new one. The same 

 happens with the rays of star-fishes ; the tail of a lizard 

 is also readily reproduced ; salamanders even possess the 

 faculty of reproducing parts of the head, including the eye 

 with all its complicated structure. Something similar takes 

 place in our own bodies, when a new skin is formed over a 

 wound, or when a broken bone is reunited. 



514. In some of the lower animals this power of repara- 

 tion is carried much farther, and applies to the whole body, 

 so as closely to imitate fissiparous reproduction. If an earth- 

 worm or a fresh-water polype be divided into several pieces, the 

 injury is soon repaired, each fragment speedily becoming a per- 

 fect animal. Something like this reparative faculty is seen in the 

 vegetable as well as in the animal kingdom. A willow branch, 

 planted in a moist soil, throws out roots below and branches 



