THE EARLY HISTORY OF THE GERM-NUCLEI 193 



rod is formed, which represents two equivalent pairs of chromosomes 



ab 



ab 



During the two maturation-divisions the four chromosomes 



are spHt apart, — nr» and Riickert's observations demonstrate that 



a\b 



the first division separates the two equivalent dyads, ab and ab, which 

 by the second division are split apart into the two separate chromo- 

 somes, a and b. Weismann's postulate is accordingly realized in the 

 second division. It is clear from this account that the primary 

 halving of the number of chromatin-rods is not an actual reduction, 

 since each rod represents two chromosomes. Ruckert therefore 

 proposes the convenient term " pseudo-reduction " for this pre- 

 liminary halving.^ The actual reduction is not effected until the 

 dyads are split apart during the second maturation-division. 



C. The Early History of the Germ-Nuclei 



We may for the present defer a consideration of accounts of reduc- 

 tion differing from the two already described and pass on to a 

 consideration of the earlier history of the germ-nuclei. A consider- 

 able number of observers are now agreed that the primary chromatin- 

 rods appear at a very early period in the germinal vesicle and are 

 longitudinally split from the first. (Hacker, vom Rath, Ruckert, in 

 copepods ; Ruckert in selachians ; Born and Pick in amphibia ; 

 Holl in the chick; Ruckert in the rabbit.) Hacker ('92, 2) made the 

 interesting discovery that in some of the copepods {Cajithocaviptus, 

 Cyclops^ these double rods could be traced back continuously to a 

 double spireme-thread, following" immediately upon the division of the 

 last generation of oogonia, and that at no period is a true retictibnn 

 formed in tJie germinal vesicle (Fig. 97). In the following year Rlick- 

 ert ('93, 2) made a precisely similar discovery in the case of selachians. 

 After division of the last generation of oogonia the daughter-chro- 

 mosomes do not give rise to a reticulum, but split lengthwise, and 

 persist in this condition throughout the entire growth-period of the 

 Qg^^. Ruckert therefore concluded that the germinal vesicle of the 

 selachians is to be regarded as a " daughter-spireme of the oogonium 

 {Ur-ei) grown to enormous dimensions, the chromosomes of v/hich 

 are doubled and arranged in pairs." ^ In the following year ('93) 

 vom Rath, following out the earlier work of Flemming, discovered 

 an exactly analogous fact in the spermatogenesis of the salamander. 

 The tetrads were here traced back to double chromatin-rods, indi- 

 vidually identical with the daughter-chromosomes of the preceding 



1 '93, 2, p. 541. - '92, 2. p. 51. 



