GERM-CELL HISTORY IN THE BROOK LAMPREY 23 



Others (Abramowicz, Bouin, Kuschakewitsch, Dustin, Firket), 

 while admitting the presence of the primordial germ cells in the 

 early embryo and the possibility that they give rise to definitive 

 reproductive products, still think it probable, and even sup- 

 ported by very strong evidence in some cases, that many of the 

 definitive germ cells are derived from other elements which, 

 strictly speaking, have formed a part of the soma. Opinions 

 vary as to whether these other cells should be considered true 

 somatic elements or simply another type of undifferentiated cells. 

 Child ('06) is convinced that in the cestode, Moniezia expansa, 

 the germ cells develop from cells of the parenchymal syncytium 

 which must be regarded as differentiated tissue cells. In his 

 development of the 'theory of dedifferentiation' ('15) he makes 

 the following statement : 



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 dediffer- 

 entiation and take part in the process. Even the large muscle cells 



may give rise to testes In such cases the muscle fiber 



undergoes degeneration, the vacuoles disappear, and the nucleus begins 

 to divide, apparently at first amitotically (pp. 331-332). 



C. W. Hargitt ('06) thinks that the germ cells in Clava leptostyla 

 arise in the entoderm, and that it is unlikely, though possible, 

 that these cells may be undifferentiated. In Campanularia 

 flexuosa, George T. Hargitt ('13) has found that the egg cell 

 arises in the entoderm by the transformation of single epithelial 

 cells, or from the basal half of divided cells. He concudes: 

 "Therefore the egg cells have come from differentiated body cells 

 (so-called) and there is no differentiation of the germ plasm in 

 the sense that the germ cells are early differentiated and set 

 aside and do not participate in the body functions" (p. 111). 



Max Jorgensen ('10) comes to the same conclusion for Sycon. 

 He says: ''Indessen zeigen mir meine Praparate dass auch eine 

 Entstehung von Oogonien aus Mesodermzellen denkbar und 

 morphologisch nachweisbar ist" (p. 169). 



b. Vertebrates. The earliest theory of the origin of germ cells 

 in vertebrates is the 'germinal epithelium theory,' advanced by 



