The Spermatogenesis of the Opossum (Didelphys virginiana) etc. 53 
plasmosome, and the enlargement of the sphere (Nebenkern). All these 
elements continue to enlarge for a time (figs. 62 to 64) but the prime 
event is here the Separation of a small plastin sphere (idiozome) front the 
still enlarging clear hyaline sphere (archoplasmic material — Nebenkern), 
'fhe latter appears to overflow the nucleus and to indent it both inwards 
and downwards and the formet - wanders to the opposite pole (figs. 62 
to 66). Meanwhile the enlarged cell has elongated, as also the nucleus. 
Coincident with these phenomena, there occurs an increase in the 
number of chondriosomes. The entire cell now seems packed with them 
(figs. 65 and 68). In cells lightly stained they appear like small vesicles ; 
ttnder deeper stain, they appear like grantües and spheres of still smaller 
granules. It seems probable also that the larger clearer vesicles are in 
process of dissolution within the cytoplasm. Duesberg (1910) describes 
similar vesictüar mitochonclria in the guinea-pig Spermatozoon of this 
stage of development. 
The nucleus continues to elongate and narrow, and ntoves to the 
surface of the cell (figs. 69 to 70). Here, and in many instances earlier, 
two clu - omatic granules (centrosomes, centrioles) appear near the smaller 
sphere (idiozonte). It seems probable that they have been iiberated by 
the sphere. The fate of this sphere is apparently elongation (fig. 82) 
and ultimate fragmentation within the body of the developing Sperma- 
tozoon (fig. 86). It probably does not contribute to the formation of 
the connecting piece since it may be entirely eliminated with the cast-off 
portion of the spermatid cytoplasm (fig. 78). The nucleus now curves 
on itself at a pole of the cell carrying the Nebenkern — the major portion 
of which now envelops the nucleus — at the forward end as an acrosome 
and takes a position approximately at right angles to its former position 
in the long axis of the cell. Meanwhile the centrosome takes its position 
on the inner surface of the curving contracting nucleus and near its centre. 
The nucleus still carries a portion of the Nebenkern (or “large sphere” 
at one pole as an acrosome. The “sphere” grows continually smaller 
(fig. 81) and may be the homologue of the acrosome of other forms. 
A section (longitudinal) of the elongate spermatid at this stage is 
illustrated in fig. 71. This represents a section through fig. 70 after 
the nucleus has become more compact and dipped downward and 
forward and rotated through an angle of approximately 90 degrees. 
Polar views of successively later stages of development are represented 
in illustrations 73 to 75. In figs. 71 and 72 the position of the centro- 
somes is clearly indicated. 
It will be noted that only a portion of the cytoplasm of the original 
