THE EMBRYOLOGY OF THE OPOSSUM 19 



is still attached to the axial filament, produces an excrescence 

 which becomes quite large and conspicuous. This later de- 

 generates, and its function is unknown. Next, the distal 

 centriole flattens and divides into a proximal granule and a 

 distal ring, the latter encircling the axial filament (fig. 3, C). 

 By this time the nucleus has assumed a curious swallow-tail 

 or V-shape, the proximal centriole being attached at the inside 

 of the angle, and the axial filament extending at right angles 

 to the plane of the V. The ring now moves distally along the 

 axial filament, and as it does so many of the chondriosomes 

 from the surrounding cytoplasm migrate to the axial filament 

 and become attached to that part over which the ring lias 

 passed (fig. 3, D). The remaining chondriosomes and the 

 remnants of the Golgi material and idiosome, together with 

 most of the cytoplasm, collect in a residual body anterior to 

 the nucleus (fig. 3, E), and are cast off. At this time the 

 spermatozoon is discharged into the lumen of the seminiferous 

 tubule. Here the chondriosomes which had become attached 

 in the middle piece fuse into a spiral around the axial fila- 

 ment (fig. 3, F) ; the V-shaped nucleus turns through an angle 

 of 90 so that the middle piece now runs between the legs of 

 the V; and, finally, the spermatozoa fuse together in pairs 

 (fig. 3, G), the line of junction being only in the head region 

 along the heavier leg of the V. These conjugate sperms, as 

 Wilson ('28, p. 305) calls them are a rare phenomenon, but 

 not unique outside of the marsupials they are found in a 

 beetle, Dytiscus (Ballowitz, 1895), and in a gastropod, Tur- 

 ritella (Retzius, '06). The significance of the pairing is not 

 known. Selenka (1887) pointed out that when the individuals 

 of a pair separate later on they can no longer swim in a 

 straight line, but describe circles. He thinks that most of 

 the spermatozoa remain paired until they have reached the 

 upper part of the oviduct, and then normally separate before 

 fertilization. 



Mating.* According to Hartman ('23 and '28), the breed- 

 ing season in Texas begins with the first week of January. 



-Tliis word is here used only as a synonym for copulation, and does not imply 

 permanent pairing. Any male will copulate with any female in oestrus. 



