416 EDMUND B. WILSON 



found to divide but once (equationally) in the course of the two 

 spermatocyte-divisions, as in case of the X-chromosomes, or the 

 ra-chromosomes . 



When these facts are taken together the conclusion seems to 

 me unavoidable that one of the divisions of the bivalent chromo- 

 some (and hence, one of the division-planes seen in the bivalent 

 prophase-figures) is a consequence of its bivalence i.e., of its origin 

 from two chromosomes instead of one, of the original diploid 

 groups. An almost conclusive demonstration of this is given by 

 the fact that when the X- and F-chromosomes are united to form a 

 bivalent in the prophases, this body, like the others, often shows a 

 tetrad structure (as in Brochymena, Wilson, '05 b, or Nezara, 

 Wilson, '11 a); and in the case of Ascaris felis, recently described 

 by Edwards ('11) this bivalent has a double cross-form, closely 

 similar to that of the other bivalents save for the inequality of 

 two of the components (op. cit., fig. 2). I do not mean to imply 

 that either division-plane of the tetrad represents the actual plane 

 of separation of the same two chromosomes that have united in 

 synapsis; on the contrary, I think it probable, as already indicated, 

 that the original chromosomes may have undergone reconstruc- 

 tion. What may be said is that one division is independent of 

 bivalence, the other a consequence of it ; and it is further clear that 

 the former effects no reduction of valence, while the latter does. 

 Whether we regard the autosome-bivalent as to its origin or its 

 fate, it has, irrespective of its relative size, double the chromo- 

 somic value of a univalent in the maturation-process; and in 

 this respect it is exactly comparable to an XF-bivalent or an m- 

 bivalent, in which one of the divisions is demonstrably a reduc- 

 tion-division in the original sense. This value, or 'valence' is 

 reduced to one-half in one of the maturation-divisions. May we 

 not here find a definition of the reduction-division that may be 

 accepted even by those who deny the individuality of the chromo- 

 somes, or who believe that synapsis is -followed by actual fusion? 

 We may define an equation-division as one that effects no reduction 

 of valence, a reduction-division as one that reduces the valence to 

 one-half. This conforms exactly to the observed facts; and such a 

 definition is, I think, equally consistent with complete disjunc- 



