350 The Spermatogenesis of Bufo Lentiginosus 
chromatin, which now stains very intensely, forms into an apparently 
continuous spireme which breaks into segments before the nuclear mem- 
brane disintegrates. There are twenty-four of these chromatin segments, 
this being the number that is characteristic of the somatic cells of the 
species. At first these segments are scattered throughout the nucleus, 
and the marked variation in their size can readily be seen. Figs. 2 and 
3 show the total number of chromatin segments in a primary spermato- 
gonium during the early prophase of mitosis; four of the segments are 
noticeably larger and longer than the rest; four others are very small; 
and the remaining sixteen segments are intermediate in size between 
these two extremes. One can readily arrange the chromosomes in twelve: 
groups in which the two members of a group lie near each other and 
are approximately of the same size. Thus the “ pairing” of homolog- 
ous chromosomes in preparation for the maturation divisions already 
exists in the original parent cells, as Montgomery (39) has stated is 
the case in Desmognathus and Plethodon. <A difference in the size of 
the chromosomes is found in the division stages of all generations of 
the secondary spermatogonia (Figs. 7-9), and also in both of the sperm- 
atocyte divisions (Figs. 43-45, 48-51, 56-57). This furnishes additional 
evidence in favor of the view advocated by Boveri, Montgomery, Sutton, 
and others that the chromosomes retain their individuality throughout 
all stages in the development of the germ-cells. After the nuclear mem- 
brane breaks down, the chromosomes condense into broad, V-shaped 
loops. Sections of the spindle during metakinesis (Figs. 4-5) show a 
marked difference in the size of the chromosomes; but at this time the 
chromosomes are so crowded together that it is impossible to determine 
whether they are arranged on the spindle fibres in any definite way. 
About the time that the chromatin spireme breaks into segments 
preparatory to the division of the cell, the centrosome divides (Fig. 3) 
and a spindle forms, evidently of extranuclear material. The spindle 
fibres converge sharply at the poles where the minute centrosomes are 
clearly seen (Fig. 5). The attraction-sphere, which always surrounds 
the centrosome during the resting stages of the cell, disappears in the 
early prophase of mitosis, as it does in the spermatogonia of Salamandra 
(Meves, Rawitz), and of Amphiuma (McGregor). It is not again visible 
until the resting stages of the daughter-cells, as the centrosomes at the 
poles of the divisicn spiudle are totally devoid of any radiation. 
