THE 



AMERICAN NATURALIST 



Vol. XLY July, 1911 No. 



GERM-CELL DETERMINANTS AND THEIR 

 SIGNIFICANCE 1 



PROFESSOR R. W. HEGNER 

 University of Michigan 



Investigations of the origin of the germ cells in a num- 

 ber of animals have brought forth certain phenomena 

 which indicate that these cells are determined as such at 

 a very early period in embryonic development, and that 

 m some cases the material which apparently determines 

 the germ cells is visible at this time. Conclusions can 

 be drawn from these observations which are of consider- 

 able interest. 



The frequently repeated statement that the germ cells 

 are derived from the mesoderm or from the entoderm 

 is of course erroneous in those instances where the germ 

 cells can be identified before the formation of the germ 

 layers, and it seems probable that the primary cell differ- 

 entiation, i. e., the separation of the germ cells from the 

 somatic cells, takes place at an early period in the em- 

 bryonic development of even those animals where this 

 has not been actually observed. A few of the most pro- 

 nounced cases of the early differentiation of germ cells 

 are briefly described in the following paragraphs and 

 several general conclusions arrived at from this evidence. 



The best known example is Ascaris, as described by 



