THE GAMETES IN PLANTS AND ANIMALS 331 



segregated from other cells at early stages of development, they do 

 not in any way constitute a valid argument for the independence 

 and continuity of the germ plasm. 



Moreover, there are many animals in which up to the present 

 time no indication of early segregation of germ cells has ever been 

 found by any investigator. In some of these forms, e.g., certain 

 flatworms and the polychete annelids, the sex organs appear only 

 at a certain stage of development, or periodically, and before or 

 between the periods of their occurrence no traces of anything 

 representing germ cells can be found. The assumption has often 

 been made that in such cases the germ plasm is segregated in cer- 

 tain cells, but that these cells possess no characteristic visible features 

 distinguishing them from other cells or tissues. In the turbellaria, 

 for example, the parenchyma has often been regarded as an "in- 

 different' 1 tissue representing the germ plasm. But the only 

 justification for terming such tissues as the turbellarian parenchyma 

 indifferent or undifferentiated tissues lies in the fact that they give 

 rise to germ cells and in reconstitution to various other parts. 

 Morphologically they are not undifferentiated, but possess definite 

 histological characteristics quite different from those of cells or 

 tissues which are really embryonic or undifferentiated, and when 

 other tissues or organs arise from them they first lose these charac- 

 teristics and become embryonic and then undergo a new differen- 

 tiation. Moreover, when they undergo such changes their rate of 

 metabolism becomes higher, an indication that they are undergoing 

 dedifferentiation and becoming younger. They may be less 

 highly specialized than certain other tissues of the organism, but 

 only theoretical grounds can prevent us from admitting that where 

 the germ cells arise from such tissues they arise from differentiated 

 functional parts of the organism by a process of dedifferentiation 

 and redifferentiation. 



In the tapeworm Moniezia, for example, the sex cells arise from 

 the parenchyma, and apparently any parenchymal cells which lie 

 within the region involved in the production of sex cells may undergo 

 dedifferentiation and take part in the process. Even the large 

 muscle cells may give rise to testes, as indicated in Figs. 146 and 147. 

 In such cases the muscle fiber undergoes degeneration, the vacuoles 



