1 iO 



THE GERM-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 



Fig. 55. — Origin of the primordial germ-cells and casting out of chromatin in the somatic 

 ceXXsoi Ascaris. [BOVERI.] 



A. Two-cell stage dividing; s. stem-cell, from which arise the germ-cells. D. The same from 

 the side, later in the second cleavage, showing the two types of mitosis and the casting out of 

 chromatin {c) in the somatic cell. C. Resulting 4-cell stage; the eliminated chromatin at c. 

 D. The third cleavage, repeating the foregoing process in the two upper cells. 



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

 cells. Haecker has recently traced very carefully the origin of the 

 primordial germ-cells in Cyclops from a "stem-cell" (Fig. 56) clearly 

 distinguishable from surrounding cells in the early blastula stage, not 



