146 



THE GERM-CELLS 



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

 a germinal epithelium which, in the cctlenterates (Fig. 72), 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 



ec 



^ B 



Fig. 72. — Origin of the germ-cells in a hydro-medusa, Cladonevia. [WeismanN.] 

 A. Young stage ; section through wall of manubrium of the medusa ; ova developing in the 

 ectoderm (ec). B. Later stage, showing older ova {o) and "nutritive cells" (ri). The ova 

 contain small nuclei probably derived from engulfed nutritive cells. 



two primordial germ-cells lying at the apex of the archenteron. In 

 some of the insects they appear still earlier as the products of a large 

 " pole-cell " lying at one end of the segmenting ovum, which divides 

 into two and finally gives rise to two symmetrical groups of germ- 

 cells. Hacker has recently- traced very carefully the origin of the 

 primordial germ-cells in Cyclops from a "stem-cell " (Fig. 74) clearly 

 distinguishable from surrounding cells in the early blastula stage, not 

 only by its size, but also by its large nuclei rich in chromatin, and by 

 its peculiar mode of mitosis, as described beyond. 



The most beautiful and remarkable known case of early differenti- 

 ation of the germ-cells is that of Ascaris, where Boveri was able to 

 trace them back continuously thiough all the cleavage-stages to the 



