180 MONTGOMERY A STUDY OF THE CHROMOSOMES 



In the later prophases (Figs. 133, 134) the five bivalent chromosomes condense into 

 the form of dumbbells or sometimes of rings, the large chromosome K. 2 (Fig. 133) 

 passing through these stages more slowly than the others, so that it often retains loose 

 texture and roughened outlines after the four others have become compact with smooth 

 outlines. The bivalent chromatin nucleolus now has its univalent components generally 

 iu rather close apposition (N. 2, Figs. 133, 134). The chromosome x is very compact 

 in structure, and when seen from the side has squarish form (x, Fig. 133), an indentation 

 at one end of which marks the point of apposition of the ends of the primitive horse- 

 shoe form which latter in some cases (Fig. 134) may still be seen at this late stage. 

 Usually the chromosome x is longer than broad, and the clear line sometimes found in its 

 long axis does not then represent the primitive longitudinal split, of which there seems 

 no trace at this stage, but the space separating the two arms of the horseshoe. 



In the inonaster stage of the first maturation mitosis (Figs. 135-137) there are 

 accordingly seven elements (in the cell from which Fig. 136 was drawn, one of the 

 bivalent chromosomes lay out of the plane of the section). These are the bivalent 

 chromatin nucleolus (N. &), the smallest of all ; the chromosome x (x), the largest of 

 all ; and five bivalent chromosomes, of which one is almost always recognizable by its 

 greater volume (K. 2), and this is the bivalent chromosome formed by the synapsis of 

 the two larger chromosomes A" of the spermatogonia. The five bivalent chromosomes 

 and the chromatin nucleolus become divided transversely in the metakinesis (reduction 

 division). The chromosome z (for successive stages in its division, Figs. 13G, 138), 

 which has its long axis coinciding with the plane of the equator, becomes divided into two 

 along its median axis. This would appear at first sight to be a longitudinal (equational) 

 division; but it is not, for we have learned that this peculiar chromosome had first t In- 

 form of a straight rod, which then bent at its middle point into a U or V, then the arms 

 of the U or V laid themselves parallel to and close together, so that a division along the 

 median axis results now in the separation of these arms, and is accordingly a reduction 

 division. 



A view of the second spermatocyte, before its chromatin elements have definitely 

 arranged themselves in the plane of the equator of the spindle, shows also seven elements 

 (Fig. 139) : one univaleut chromatin nucleolus (N. 2) ; one chromosome larger than the 

 others, a half of the original chromosome x (x i); and five univalent chromosomes, one 

 (A") larger than the others and directly comparable to one of the large chromosomes A of 

 the spermatogonia. In the metakinesis following (Fig. 140) the five univalent chromo- 

 somes are divided equationally, and the univalent chromatin nucleolus (N. %) is also 

 divided (but in what plane was not determined). The chromosome x \, however, never 

 becomes divided in this mitosis, but passes undivided into one of the daughter cells (Fig. 



