THE SPECIFIC FORM. 207 



evacuation of faeces, and the rhythmical contrac- 

 tion of the pulsatile vesicules are concerned. But 

 Balbiani's researches in 1892 have shown us that 

 secretion, complete regeneration, and the faculty of 

 reproduction by fission can take place only in the 

 nucleated fragment — i.e., in the nucleus. 



Accidental Reproduction in the Metazoa. — Among 

 multicellular beings the faculty of reproduction is met 

 with in the highest degree in plants, where we find it 

 in the process of propagation by slips. In animals it 

 is the most marked in Ccelenterata. Trembley's 

 experiments are a striking instance. We know that 

 when the hydra is cut into tiny pieces it reproduces 

 exactly as many complete beings. Among the 

 worms, Planaria afford a similar example. Every 

 fragment, provided its volume is not less than a tenth 

 of that of the whole, can reproduce a complete, entire 

 being. The snail can produce a large part of its head, 

 including the tentacles and the mouth. Among the 

 Tritons and the Salamanders the faculty of regenera- 

 tion reproduces the limbs, the tail, and the eye. In 

 the Frog family, on the contrary, the work of re- 

 generation does not go beyond cicatrization, and it is 

 the same with Birds and Insects. 



It is really startling to see in a vertebrate like the 

 Triton the stump of an arm with its fragment of 

 humerus reproducing the forearm and the hand in all 

 their complexity, with their skeleton, blood vessels, 

 nerves, and teguments. We say that the limb has 

 budded, as if there were a germ of it which develops 

 like the seed of a plant, or as if each transverse portion 

 of the limb, each slice, so to speak, could re-form the 

 slice that follows. 



The mechanism of generation and that of regenerjgi' 



