THE SPONGES— PHYLUM PORIFERA 113 



Regeneration 



There is another process which sponges can carry on that may be 

 discussed in connection with reproduction. This is regeneration, which 

 is not a normal means of reproduction, but on occasion may serve to 

 produce new individuals and, at least, we may refer to it as an acci- 

 dental means of reproduction. When a scypha is cut into two parts, 

 each part will proceed to regenerate the missing piece so that two com- 

 plete animals will be formed. The same will be true if cut into four 

 parts and may even be carried so far that a small piece cut out of the 

 animal will regenerate a complete new body. 



We can demonstrate the remarkable powers of regeneration in 

 sponges by forcing the pieces of a red sponge (Microciona) through 

 heavy silk bolting cloth. This breaks the sponge into separate cells, 

 or groups of very few cells, but these gradually clump together in the 

 bottom of the dish and form numerous small sponges. In a short 

 time the flagella of the collar cells begin to beat, and each clump becomes 

 a functioning individual. 



This great power of regeneration is gradually lost in the animal king- 

 dom as the complexity of the animals increases. Because of the speciali- 

 zation of different parts of the body for different functions, the cells have 

 to become more highly differentiated from the embryonic cells from 

 which they came, so that the embryonic function of reproducing all types 

 of cells is lost. For instance, the highly complex nerve cell of man has 

 gone so far in adapting itself to its particular function that it is incon- 

 ceivable that it could regenerate skin, bone, or muscle. The power is 

 never completely lost, however, because, even in man, new skin cells can 

 be regenerated to replace those destroyed ; new bone and muscle cells are 

 regenerated from injured bone and muscle tissue, and so on. The power 

 seems to have reached its lowest ebb in nerve tissue, probably because it 

 contains the most specialized of all cells. Cells of the brain and spinal 

 cord can never be regenerated if destroyed ; permanent insanity will re- 

 sult from destruction of a part of the brain by disease or injury. How- 

 ever, processes of the nerve cells that run from the brain and spinal 

 cord to various parts of the body can be regenerated if the part contain- 

 ing the nucleus is not damaged and the regenerating process has some- 

 thing to guide it. 



Scypha, on the other hand, has all of its cells very much alike and 

 each carries on the various life processes, so that each cell has about the 

 same power to produce all types of cells necessary to form a new animal 

 as did the zygote or the embryo. Somewhere between these two ex- 

 tremes we find animals like the earthworm in which, if cut in two, the 



