136 



CYTOLOGY 



CHAP. 



traced through the gradually widening out V into the rod, where it 

 persists as a constriction, or an actual break in the chromatin, dividing 

 the chromosome into two portions connected by a Hnin bridge. This 

 transverse constriction is the same as that so characteristic of the meiotic 

 bivalents which causes them to appear in " tetrad " form as discussed 

 on p. 40. The significant fact is that the transverse break always 

 occurs in the same region in the same chromosome. It will be noticed 

 from Fig. 65 that one pair of chromosomes is much larger than any of 

 the others. The break in this chromosome — whether exhibited as the 

 angle of the V, or as a transverse constriction — ^is always found at about 



BCD 



1% 



f 



Fig. 65. 



It 



-^ « 



The eight largest chromosomes of Lepidosiren. (Agar, Q.J. M.S., 1913.) A, from a spermatogonia! anaphase ; 

 B, the four bivalents formed by the pairing of the eight chromosomes (late prophase I.) ; C, metaphase I. ; D, 

 anaphase I., each univalent split for the second division. Note that chromosomes i and 2 form a V with equal 

 limbs in A, and that each constituent of the corresponding bivalent is similarly constricted into equal portions. 

 The other three pairs of chromosomes have unequal limbs, both in A and in the bivalents. 



the middle of the chromosome. The next two pairs of chromosomes are 

 of much the same size, but easily distinguishable both from the large 

 pair just described and from the next smaller pair. These two pairs 

 constantly have the break excentrically placed. Now if the break 

 were always in the middle of the chromosome, or varied in position in 

 the same chromosome, it would be without significance for the present 

 purpose, as it might then be due to purely accidental mechanical causes. 

 The fact that — however caused — it is constant in position in a given 

 chromosome, but differs in different ones, indicates that the chromosomes 

 possess a constant differentiation in a lengthwise direction. 



Wenrich (1916) has found that in the prophase chromosomes of 

 the Orthopteran, Phrynotettix magna, the principal chromomeres are 



