\\'hitine-: Cvtoloo'v and Genetics 



117 



clearly shown paras\napsis of very 

 fine threads, but in view of the diffi- 

 culty of fixation in much botanical 

 material it is not surprising that this 

 phenonemon has not been more \videl\' 

 observed. 



Structure of First Maturation 

 Chromosomes 



The number of elements, chroma- 

 tids, present in the chromosomes of 

 the heterotypic division seems to be a 

 bone of contention among cytt)log"ists. 

 Zoologists have for the most ])art, at 

 least recently, held that there are four, 

 and have hence designated these 

 chromosomes as tetrads. Botanists, as 

 in the case of Gates, perhaps because 

 their material is not so easily fixed, 

 have usually been unable to see more 

 than two elements at this stage, and 

 have consequently preferred the term, 

 bivalent chromosomes. Recentlv, how- 

 ever, Taylor' in a preliminary report 

 on the organization of heterotypic 

 chromosomes of the Liliaceous genus 

 Gasteria, states that clefts appear in 

 the ends of the metaphase chromo- 

 somes, separating monads, and that 

 the latter are completely separated at 

 anaphase. The conclusion is therefore 

 unavoidable that in this plant material 

 at least the firsi: maturation chromo- 

 somes are tetrads in structure. The 

 term bivalent may well be retained, 

 nevertheless, to denote origin. 



Failure to reveal the tetrad nature 

 in many cases is evidence merely that 

 the chromatids tend to associate 

 much more closely in pairs than the 

 pairs associate with each other. To 

 use a chemical analogy, we might say 

 that the valence of each chromatid is 

 more or less satisfied by very close 

 union with one other chromatid. 

 Homologous or sister pairs may then 

 be more loosely associated giving the 

 appearance of double elements which 

 are really quadruple. 



Structure of Spirene Chromosomes 



A comparable situation is to be ob- 

 served in spireme stages, preceding 

 maturation. It is often stated that the 



homologous chromosomes twist about 

 each other in long threads. This is 

 only partly true as 1 have pointed out 

 in the spei-matogenesis of Culex." Two 

 twisted chromatin threads indeed cip- 

 l)ear, but each of these threads con- 

 sists in any loop of two tightly fused 

 elements, chromatids. While the 

 threads in one loop may represent por- 

 tions of homologous chromosomes, 

 the threads of the next loop may repre- 

 sent fused daughters of homologues. 

 In other words the chromatids have 

 changed partners at the apparent point 

 of junction. The spireme chromo- 

 some then is really a tetrad, for it is 

 made up of four elements. 



Spireme Structure and Crossing Over 



The fundamental quadruple nature 

 of the early spireme thread is of im- 

 portance in relation to genetics. 

 Bridges' (p. 134) from a study of 

 equational non-disjunction has shown 

 that non-cHsjunction of the equational 

 and of the ordinary reductional tvpe 

 may occur at the same time in the 

 same "pair" of chromosomes. This 

 necessitates crossing over in the four- 

 strand stage. "It is impossible that a 

 non-cross over and a cross over 

 chromosome come from a cell in which 

 only two strands are present. There- 

 fore the proof that these exceptions 

 arise from XX (or XX Y) oocytes 

 would at the same time prove that 

 crossing over took place in this man- 

 ner at a four strand stage." But since 

 Plough's results show crossing over 

 to occur very early, we will perhaps 

 have t(j conclude that even the zygo- 

 tene threads, although apparently dou- 

 ble, are really quadruple, the double 

 appearance being due to the same 

 jjrinciple of "satisfied valence" which 

 causes the later spireme also to appear 

 double and the diakinesis tetrads in 

 some cases to appear as "dyads." 



Robertson' has reported precocious 

 splitting of anaphase chromosomes in 

 the last spermatogonial division, a 

 cytological fact which may be taken 

 as evidence of double nature of lep- 



