H. E. Jordan 
the second centrosome could not be determined. It is probably repre- 
sented by the plate separating middle-piece from flagellum (text fig. 2). 
An axial tail filament extends from the end-knob throngh the middle- 
piece to the tip of the flagellum. There is no distinct end-piece at this 
stage. Analogy in a number of points with conditions described by Meves 
(1908) and Duesberg (1910) for the guinea-pig and by Duesberg (1908) 
for the rat may hold also with respect to the middle-piece of the Opossum. 
If so, it is probably formed by the second (wandering or migratory) cen- 
trosome and surrounded by the mitochondrial sheath (spiral filament). 
The latter appears separated from the axial filament by an appreciable 
interval and enclosed in a delicate superficial membrane. The body 
proper comprises that portion of the original cytoplasm of the spennatid 
enclosed within the limits of that part of the X ebenkern which flows 
around and beliind the nucleus. A small portion, persisting anteriorlv, 
may form an acrosome. 
Literature. Tire only literature touching the morphology of the 
opossum Spermatozoon (spermion ; zoosperme) is a brief description, with 
several illustrations of paired spermatozoa from the vas deferens, by 
Selenka (1887). As regards structure simply, I can add nothing essentiallv 
new to Selenka’s earlier illustrations. 
Fürst (1887) has described the structure of the Spermatozoon of a 
marsupial (MetacJiirus quica). This seems to differ only in unimportant 
details from Selenka’s description of the opossum Spermatozoon. The 
U-shaped head, Fürst divides into two “partes laterales”, “crura capitis” 
(free ends) and “pars intermedia capitis” (thicker curved end). More- 
over, Fürst rcverses ventral and dorsal sides according to my Interpre- 
tation of the morphology of the opossum sperm. Fürst thinks that the 
acrosome is applied externallv to the bend of the “U”. The spermato- 
histogenetic process of the opossum, as above described, shows clearly, 
I believe, that the acrosome or its analogue, is placed at the opposite 
pole (i. e. in opening of the “U”). Neither Fürst nor Selenka have 
studied the development of the marsupial Spermatozoon tliey describe, 
nor has marsupial spermatogenesis, as far as I can learn, ever been de- 
scribed in full. Von Korff (1902) gives a brief description of the Sper- 
matozoon of the kangaroo. 
Regarding the origin and proximate fate of the mitochondria, X eben- 
kern and idiozome a brief comparison will be made in the next section 
between the spermatoozon of the opossum and that of the rat (von Len- 
hossek, 1898; Retzius, 1909; Regaud, 1908; and Duesberg, 1908), 
guinea pig (Meves, 1896, 1899; Duesberg, 1910), cat (Leplat, 1910). 
