Tlie Spermatogenesis of the Opossum (Didelphys virginiana) etc. 
49 
e) Prophase of First Maturation Division (reductional). 
Haploid (simplex) Group of Chromosomes. 
The typical prophase chromosomes consist of dehcate rings, loops 
and rods. Typical tetrads are not present. Among the autosomes the 
spherical or oval monosome is conspicuous (figs. 25 to 28). In terms of 
a loop (the original form of the bivalent chromosome — figs. 15, 16 and 18 — 
apparently not materially modified during the postsynaptic phase) the 
rings are closed loops, and the pairs of rods are loops opened at the bends 
(the synaptic points). This point will be further discussed in the follo- 
wing sub-section. 
The diploid number of chromosomes is 17. The haploid number 
is 9 (figs. 29 to 34). Metaphase plates of chromosomes are frequently 
in the form of a U with the large bipartite accessory chromosome near 
one end (figs. 29 to 32). This larger odd chromosome can be identified 
by its size and shape during all the successive stages of the first maturation 
mitosis (figs. 29 to 38). The prevailing form of the chromosomes of these 
equatorial plates is either oval or dumb-bell-or double-rod-shape. The 
ordinary chromosomes (autosomes) vary only slightly in size. The haploid 
number 9 is interpreted as due to a pairing of 16 of the diploid group 
plus one unpaired or accessory chromosome. 
f) Metakinesis. 
During metakinesis the larger unpaired bipartite odd chromosome 
passes undivided, and in advance of the ordinary chromosomes, to one 
pole (figs. 34 to 37). It can still be distinguished at the late anaphase by 
its larger size (fig. 38). Since a number of the chromosomes in the meta- 
phase plates are distinctly of the form of paired rods, and some of the 
chromosomes on the spindle are of this form, and are simply drawn apart 
in their passage towards opposite poles, this is regarded as the type to 
which the ovals and dumb-bells may be reduced. If then these represent 
the original double threads of the prophase (simply less compact) — an 
inference which seems amply justified by appearances — i. e., the synaptic 
loops opened at the point of union, then this first division separates whole 
chromosomes and is a reducing division. The result of metakinesis is 
an anaphase figure with 9 chromosomes at one pole (8 autosomes + a 
monosome) and 8 at the other, giving rise from this point of view to a 
dimorphism of secondary spermatocytes. 
Archiv f. Zellforschung. VII. 
4 
