Reproduction and Development. 41 



of differentiation is different. These organisms are com- 

 posed of many cells ; and instead of the parts of the cell 

 differentiating in several directions, the several cells differ- 

 entiate each in its own special direction. This is known 

 as the physiological division of labour. The cells merge 

 their individuality in the general good of the organism. 

 Each, so to speak, cultivates some special protoplasmic 

 activity, and neglects everything else in the attainment of 

 this end. The adult metazoan, therefore, consists of a ! , 

 number of cells which have diverged in several, sometimes 

 many, directions. 



In some of the lower metazoans, reproduction may be 

 effected by fission. Thus the fresh-water hydra is said to 

 divide into two parts, each of which grows up into a perfect 

 hydra. It is very doubtful, however, whether this takes 

 place normally in natural life. But there is no doubt that 

 if a hydra be artificially divided into a number of special 

 pieces, each will grow up into a perfect organism, so long 

 as each piece has fair samples of the different cells which 

 constitute the body-wall. Sponges and sea-anemones may 

 also be divided and subdivided, each part having the power 

 of reproducing the parts that are thus cut away. When 

 a worm is cut in half by the gardener's spade, the head 

 end grows a new tail ; and it is even stated that a worm 

 not only survived the removal of the first five rings, in- 

 cluding the brain, mouth, and pharynx, but within fifty- 

 eight days had completely regenerated these parts. 



Higher up in the scale of metazoan life, animals have 

 the power of regenerating lost limbs. The lobster that 

 has lost a claw reproduces a new one in its stead. A snail 

 will reproduce an amputated "horn," or tentacle, many 

 times in succession, reproducing in each case the eye, with 

 its lens and retina. Even a lizard will regenerate a lost 

 tail or a portion of a leg. In higher forms, regeneration 

 is restricted to the healing of wounds and the mending of 

 broken bones. 



Closely connected with this process of regeneration of 

 lost parts is the widely prevalent process of reproduction 



