. Cytology, with special reference to the metazoan nucleus. Cells. 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 linin 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
. Cytology, with special reference to the metazoan nucleus. Cells. 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 linin 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. Fig. 65. The eight largest chromosomes of Lepidosiren. (Agar, , 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 1 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
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