ORIGIN OF THE TETRADS 



189 



pods by Riickert, Hacker, and vom Rath, in the frog by vom Rath, 

 and in elasmobranchs by Moore. The genesis of the ring was first 

 determined by vom Rath in the mole-cricket (Gryllotalpa, '92), and 

 has been thoroughly elucidated by the later work of Riickert ('94) 

 and Hacker ('95, i). All these observers, excepting Wilcox and 



Fig. 94. Formation of the tetrads and polar bodies in Cyclops, slightly schematic. (The 

 full number of tetrads is not shown.) [RticKERT.] 



A. Germinal vesicle containing eight longitudinally split chromatin-rods (half the somatic 

 number). D. Shortening of the rods; transverse division (to form the tetrads) in progress. 

 C. Position of the tetrads in the first polar spindle, the longitudinal split horizontal. D. Ana- 

 phase ; longitudinal division of the tetrads. E. The first polar body formed ; second polar 

 spindle with the eight dyads in position for the ensuing division, which will be a transverse or 

 reducing division. 



Moore (see p. 201), have reached the same conclusion; namely, that 

 the ring arises by the longitudinal splitting of a primary chromatin- 

 rod, the two halves remaining united by their ends, and opening out 

 to form a ring. The ring-formation is, in fact, a form of heterotypi- 

 cal mitosis (p. 60). The breaking of the ring into four parts involves 



