112 



GENERAL EMBRYOLOGY 



tions in the body wall or through special tubes or ducts. In 

 the testes these ducts may lead directly to the outside from the 

 cavities of the gonads. The structure of the gonad can nearly 

 always be reduced to that of a complexly folded epithelium 

 the essential elements in which are the germ cells; the coelomic 

 surface is the free surface of the germinal epithelium. This 

 relation becomes important in describing the fundamental 

 morphology of the germ cell. 



It is a question whether the germ cells are to be considered as 

 originally undifferentiated cells, which become modified during 

 the life of the organism for the reproductive function, or 

 whether they are set apart from the beginning of the organism's 

 multicellular existence as reproductive cells, and become visibly 

 modified only in later stages. In those few forms where the 



FIG. 59. A. Section through an early embryo of the Teleost, Micrometrus 

 aggregatus, showing the distinct germ cells. After Eigenmann. ec, ectoderm; 

 en, endoderm; g, germ cells;" so, somatic layer of mesoderm; sp, splanchnic layer 

 of mesoderm. B. Section through forty-cell stage of the Crustacean, Cyclops 

 brevicornis, showing, g, the cell that gives rise to the germ cells, en, the primi- 

 tive endoderm cell in the process of its first division. After Hacker. 



germ cells are diffused it seems that any tissue cell which is not 

 completely specialized in some other direction may assume 

 reproductive characters. In forms which develop special 

 gonads, however, there are many reasons for believing that the 

 grm cells are always to be distinguished as such very early in 

 the history of the individual organism. In A scans the history 

 of the primordial germ cells 'has been traced back to one of the 

 two cells resulting from the very first division of the fertilized 

 ovum; not all of the descendants of this cell are germinal how- 



