40 CYTOLOGY 



CHAP. 



contraction, reserving syndesis for the chromosome conjugation. Others 

 use synapsis for the conjugation and synizesis for the contraction. In 

 this book the word synapsis is not used at all, on account of the confusion 

 as to which of the phenomena included in the original description it 

 should be reserved for. The term synizesis is employed throughout for 

 the contraction and syndesis for the chromosome conjugation. 



That separation of the constituents of the pachytene bivalents which 

 occurs in the diplotene stage is carried to a much greater extent in 

 Lepidosiren than in Tomopteris, so that by the time diakinesis is reached 

 we frequently have all, or all except two or three, of the bivalents resolved 

 completely into their univalent constituents (Fig. i6a, K). Examination 

 of the stages leading up to this shows that in the beginning of the diplotene 

 stage the bivalents separate first in the middle, remaining attached for a 

 time at their ends and thus form long oval rings. Subsequently these 

 rings break apart at the points of contact of the ends of their component 

 chromosomes. In diakinesis we thus find thirty-eight univalents, or at 

 least a majority of univalents with a few bivalents of which the com- 

 ponents have not completely separated. 



Since in metaphase I. we again find nineteen bivalents, the chromo- 

 somes must reunite before this stage is reached. This second pairing 

 is apparently effected by the recently separated homologous chromosomes 

 coming into contact again, first by one end and later by the other, to 

 form closed rings similar to those present in the diplotene nucleus, with 

 the difference that the rings are now much smaller and thicker. In the 

 nucleus shown in Fig. i6a, L, there are fifteen complete rings, three pairs 

 of univalents are in contact by one end only (one of these being the 

 large pair) and two are still unpaired, accounting for the thirty-eight 

 chromosomes in all. 



A conspicuous feature which now presents itself is the transverse 

 constriction or joint which is to be seen across each univalent chromo- 

 some. Sometimes (in the longer chromosomes) this takes the form of a 

 sharp angle in the chromosome, in others (the shorter ones) it is merely 

 a deep constriction making the chromosome dumb-bell shaped. The 

 result of each univalent being constricted across in this way is to make 

 each bivalent appear tetrapartite. 



The development of these transverse constrictions can be traced by 

 gradual stages which space prevents from figuring here. It is not 

 apparent in the long chromosomes which are first formed by the disjunc- 

 tion of the bivalents in the diplotene stage, but as they contract it 

 gradually makes its appearance. An exactly similar process is to be 

 observed in the contraction of the long chromosomes of the pre-meiotic 

 metaphase, as they recede towards the poles in anaphase (Fig. i6, D). 



