MULTIPLICATION IX AXLAIALS 



483 



care as would be used on human beings, surgeons are learning how to 

 perform such operations safely. It has been supposed that it may 

 some day be possible to replace a diseased kidney, for example, with one 

 taken from a sheep or from another person who has recently died or 

 who has had to have his kidney removed for some reason. This principle 

 is now applied in the repairing of crushed bones. The injured part is 

 neatly cut away and the missing portion is replaced with a suitable piece 

 of the same size taken from one 

 of the large bones that can spare 

 it or from the leg of a sheep. The 

 chief obstacle to the practical use 

 of grafting on human beings and 

 other warm-blooded animals lies 

 in the fact that the protoplasm, 

 especially of the white corpuscles, 

 develops substances that are antag- 

 onistic to foreign proteins (see 

 Chapter XXXIII). 



354. Spores. A number of 

 one-celled animals related to 

 the ameba produce spores in a 

 manner similar to that of plants. 

 The number of spores is usually 

 very large. In the malaria Plas- 

 modium spores are produced 

 while the parasite is in the red 

 corpuscles of the host. When 

 the mass of protoplasm has 



grown to its limit, it breaks up into a large number of pieces, 

 and these are thrown into the blood plasma. The chill takes 

 place when the spores are being discharged. 



355. Sexual reproduction among invertebrates. In all classes 

 of animals reproduction may occur by the sexual method, al- 

 though there are some species in which only asexual reproduc- 

 tion occurs. Among the invertebrates simple conjugation is 

 found among the protozoa (see Fig. 202). Among the coelenter- 

 ates, sponges, starfish, clams, and oysters, fertilization usually 

 takes place in the water, into which the male discharges 



Fig. 201. Regeneration in starfish 



The mutilation of starfish does not seem 

 to kill them, for each part may regrow 

 enough to complete a new individual. 

 The regenerated ray shown in the fig- 

 ure is smaller than the rest, but in time 

 the new ray would become full size, 

 since regenerating tissues and organs 

 grow faster than uninjured parts 



