ORIGIN OF THE TETRADS 



249 



in copepods by Riickert, Hacker, and Vom Rath, in pteridophytes by 

 Calkins and Osterhout, in the onion, Allium, by Ishikawa, and in 

 various other forms where their history has been less clearly made 

 out. The genesis of the ring was first determined by Vom Rath in 

 the mole cricket {Gry I lotalpa, '92), and has been thoroughly elucidated 

 by the later work of Riickert ('94), Hacker ('95, i), and Paulmier 

 ('99). All these observers 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 



D E F 



Fig. 122. — Origin of the tetrads by ring-formation in the spermatogenesis of the mole-cricket 

 Gryllotalpa. [\"OM RaTH.] 



A. Primary spermatocyte, containing six double rods, each of which represents t%vo chromo- 

 somes united end to end and longitudinally split except at the free ends. B. C. Opening out of 

 the double rods to form rings. D. Concentration of the rings. E. The rings broken up into 

 tetrads. F. First division-figure established. 



heterotypical mitosis (p. 86). The breaking of the ring into four 

 parts involves, first, the separation of these two halves (corresponding 

 with the original longitudinal split), and second, the transverse division 

 of each half, the latter being the reducing division of Weismann. 

 The number of primary rods, from which the rings arise, is one-half 

 the somatic number. Hence each of them is conceived by Vom Rath, 

 Hacker, and Riickert as bivalent or double ; i.e. as representing two 

 chromosomes united end to end. This appears with the greatest 

 clearness in the spermatogenesis of Gryllotalpa (Fig. 122). Here 



