GERM-CELL HISTORY IN THE BROOK LAMPREY 27 



reproductive cells in animals. Table 3 presents a summary of the 

 various theories relating to the origin of the germ cells in 

 vertebrates. 



For one who adheres closely to the germ-plasm theory it is 

 hard to conceive of the germ cells as coming from any other 

 cells than the early segregated embryonic cells which have had 

 no part in the building up of the body. Some who believe in the 

 theory of early segregation maintain that, even when the germ 

 cells appear to arise from so-called somatic elements, these are in 

 the strict sense not somatic cells, but cells that have maintained 

 their embryonic structure and have not specialized in any given 

 direction. There is, of course, no direct evidence for this. In 

 many forms, it is true, the germ cells seem to be segregated 

 very late in the life of the individual. This is apparently true 

 of annelids and flatworms among animals, and it seems to be true 

 of all plants. There are no investigators of germ cells in verte- 

 brates who maintain that they come from highly differentiated 

 somatic cells, such as muscle cells, as observed by Child ('06) 

 in Moniezia. 



The various theories concerning the origin of germ cells in 

 vertebrates have now been stated. Below is a partial list of the 

 most important contributions on the subject, each followed by a 

 brief statement of the conclusions which the various investigators 

 have reached. The references have been arranged chronologi- 

 cally for the various groups of vertebrates. 



