108 



FUNDAMENTALS OF CYTOLOGY 



this account will be best sei-ved by confining attention largely to those 

 features more or less common to all of them. 



Leptotene Stage. — The chromonemata, after having presented a rather 

 confused picture since the premeiotic telophase, become more distinct 

 from one another in the early meiotic prophase and appear as very long 

 and slender threads. They are present in the diploid number, and each 

 represents a chromosome. Because of their great attenuation and the 

 scarcity of enveloping matter, their chromomeres show plainly. Whether 

 their singleness is actual or only apparent because of their slenderness is a 

 debated point. In some meiocytes, notably in animals, the threads may 

 all be oriented with one end toward the same side of the nucleus, forming 

 a so-called bouquet (Fig. 77). 



Zygotene Stage. — The leptotene threads now become very closely 

 associated laterally in pairs, each of them cohering, though not actuall>' 



Fig. 77. — Stages in meiosis in spermatocyte of salamander. 1, leptotene threads developing: 

 2, pachytene stage; 3, diplotene stage; 4, metaphase I. 



fusing, with its homologue (Fig. 78). This selective pairing, or synapsis, 

 begins at one or more points, often at the ends or the kinetochore, and 

 gradually extends "zipper-hke" until it is complete. The extension and 

 the slenderness of the chromonemata, which reach their maximum length 

 at leptotene-zygotene, are believed to bear a causal relation to the synaptic 

 union. In some nuclei, notably in those with threads arranged in a 

 "bouquet," one portion may be occupied by paired threads while the rest 

 shows only threads still unpaired ; this is the amphitene condition. As the 

 threads pair, they immediately begin to shorten and thicken. 



It appears likely that synapsis is facilitated by the arrangement 

 assumed by the chromosomes at the close of the last premeiotic anaphase. 

 All the members of the complement then lie more or less parallel, with 

 their kinetochores directed toward the pole, so that in the resulting 

 telophase nucleus their arrangement is not a haphazard one. Further- 

 more, in some organisms a loosel}^ paired arrangement of homologues is 

 evident during the premeiotic mitoses or even earlier (page 99). 



Pachytene Stage. — After synaptic pairing has been completed, the 

 nucleus is said to be in the pachytene stage, for the threads are then 



