ORIGIN AND GROWTH OF THE GERM-CELLS 



IO9 



temperature, females ; while those of Mrs. Treat on lepidoptera and 

 of Yung on amphibia seem to leave no doubt that the differentiation 

 here depends on the character of the nutrition, highly-fed individuals 

 producing^a great preponderance of females, while those that are 

 underfed give rise to a preponderance of males. These and a multi- 

 tude of related observations by many botanists and zoologists render 

 it certain that sex as such is not inherited. What is inherited is, in 

 Busing's words, only the particular manner in which one or the other 

 sex comes to development. The determination of sex is not by in- 

 heritance, but by the combined effect of external conditions. 1 In 

 some of the rotifers, however, sex is predetermined from the begin- 



Fig. 54- Germ-cells in the hydro-medusa, Hydractinia. [BUNTING.] 



A. Section through young medusa-bud, with very young ova (ov.) lying in the entoderm; 

 B. Mature gonophore, showing two ova lying between ectoderm and entoderm. 



ning, the eggs being of two sizes, of which the larger produce females ; 

 the smaller, males. 



In the greater number of cases, the primordial germ-cells arise in 

 a germinal epithelium which, in the ccelenterates (Fig. 54), may be a 

 part of either the ectoderm or entoderm, and, in the higher types, is a 

 modified region of the peritoneal epithelium lining the body-cavity. 

 In such cases the primordial germ-cells may be scarcely distinguish- 

 able at first from the somatic cells of the epithelium. But in other 

 cases the germ-cells may be traced much farther back in the develop- 

 ment, and they or their progenitors may sometimes be identified in 

 the gastrula or blastula stage, or even in the early cleavage-stages. 

 Thus in the worm Sagitta, Hertwig has traced the germ-cells back to 



1 See Diising, '84; Geddes, Sex, in Encyclopedia Britannica ; Geddes and Thompson, 

 The Evolution of Sex; Watase, On the Phenomena of Sex-differentiation, '92. 



