288 



EVOLUTION AND ANIMAL LIFE 



gists who believe with Weismann that there is a sharp distinc- 

 tion between the germ plasm and the somatic or body plasm, 

 and that this germ plasm is limited to the germ cells and 

 germ-cell producing tracts, the regeneration of a nearly whole 

 body or even a considerable part of a body from a region which 

 does not include a germ cell presents a serious obstacle. But 



before this obstacle 

 can be considered as 

 one rendering the 

 germ plasm theory 

 absolutely untenable, 

 it is necessary to 

 prove what the re- 

 generated parts are 

 composed of. Are 

 they composed sim- 

 ply of repeated simi- 

 lar cells, all of one 

 tissue type, or do 

 they include other 

 kinds of cells or tis- 

 sues than those par- 

 ticular kinds from 

 which the regener- 

 ated part springs? 

 It is, of course, ad- 

 mitted that many, 

 indeed most cells of 

 the body, can repro- 

 duce other cells like themselves. Now is it a fact that regen- 

 erated parts are composed of different kinds of cells? As a 

 matter of fact this has been proved to be so by observation 

 and by experiment. Numerous instances are known in which 

 body cells arising originally from one germ layer have pro- 

 duced in the course of regeneration not only cells like them- 

 selves, but others which in normal development could only 

 arise from another germ layer. So it is plain that the study 

 of regeneration has already done much to modify our former 

 conceptions of the factors and conditions of development. 



FIG. 172. Regeneration of the blastula and gastrulse 

 of sea urchins; line indicates where the blastula or 

 gastrula was cut in half; the smaller figures show re- 

 sults of the regeneration of the two halves of each. 



