The Spermatogenesis of tlie Opossum (Didelphys virginiana) etc. 
55 
X. Summary of Results, Review of Literature and Discussion respecting : 
a) A.ceessory Chromosome. 
Summary. In the nuclear cycle of the male germ-cells of the 
opossum an odd or unpaired (differential) chromosome appears com- 
parable in form and behavior to the heterotropic or accessory chromosome 
of the Tracheata. It is probably the larger member of the spermatogonial 
(duplex) group of 17. It persists as a chromatin- (chromosome) nucleolus 
in the resting phase of the primary spermatocyte ; is conspicuously present 
throughout synapsis and among the prophase chromosomes of the first 
division; is recognizable as the large bipartite member of the reduced 
(simplex) chromosome group; passes undivided and in advance of the 
ordinary chromosomes to the pole; persists as a chromatin nucleolus in 
the brief resting stage of the secondary spermatocyte; divides like the 
ordinary chromosomes in the second maturation mitosis ; and then' deter- 
mines a dimorphism of spermatids, one type with 5 chromosomes, the 
other with 4 (which resolve into 9 and 8 respectively) ; and is lost (per- 
manently for the spermatogenic process) in the reticular phase of the 
resting spermatid. 
Literature. The accompanying bibliography includes numerous 
papers in which the literature of the accessory chromosome in inverte- 
brates is critically and extensively reviewed. Only Guyer has thus far 
reported accessory chromosomes for male vertebrates including pigeon 
(1903), rooster (1909b — probably triple), guinea fowl (1909a), man (1910 — 
double) and rat (1910). It seems probable that the analogue of the acces- 
sory chromosome of insects may be found in a wide ränge of vertebrates 5 ). 
Recently such an element has been reported by Winiwarter and Saint- 
mont (1909) in the ovary of the cat. 
Discussion. One of the most interesting phenomena in the sperma- 
tohistogenesis of the opossum is the occurrence of a double numerical 
reduction in the secondary spermatocytes (hemioid group). A similar 
phenomenon has already been described by von Bardeleben (1898) 
for man. A similar condition has been reported by Guyer in pigeon 
(1903), rooster (1909a), guinea-fowl (1909b) and man (1910). The meaning 
of this phenomenon is obscure. One might indulge in endless specu- 
lation and spin varions theories, all of which seem gratuitous since, due 
to the resting phase ordinarily intervening between the first and second 
maturation mitosis, we have no knowledge as to the value, in terms of 
univalent chromosomes, of the bipartite elements of the hemioid group. 
It seems more probable that the bipartite character simply foreshadows 
