The Spermatogenesis of the Opossum (Didelphys virginiana) etc. 
51 
Similarity of form between the chromosomes of the first and 
second metaphase plates (i. e. double rods) suggests a similar manner of 
division: accordingly a second reduction. When one recalls, however, 
that a resting stage (figs. 39 to 41) usually intervenes between the first 
and second maturation divisions, when the chromosomes pass through 
a reticular phase, the above conclusion is inadmisible; or rather, no de- 
finite conclusion respecting the character of the second division is justified. 
A double true reduction, suggested by the form of the chromosomes is 
contrary to our fundamental conceptions regarding the significance of 
chromosomes and need not, in view of the nature of the evidence, be 
seriously considered. Moreover, in the stage just preceding the brief 
resting phase of the spermatids (figs. 57, 58 and 59) there occurs a reso- 
lution of the 5 chromosomes into 9 and of the 4 into 8. This demonstrates 
that the true character of the second division is equational. The second 
numerical reduction involves a less close union apparently than the first, 
as a comparison of illustrations 29 and 43 will show. Again the fusion is 
sometimes incomplete to the extent of giving an occasional count of 
6 chromosomes. 
VII. Spermatids (dimorphic). 
Accordingly, there results a dimorphism of spermatids, one type 
containing 9 chromosomes (8 autosomes plus 1 monosome), the other 
8 (autosomes). If the demonstration can be made, as seems likely on 
grounds of analogy, that the female has 18 chromosomes there is here 
another case to which Wilson’s (1906) formula for sex-determination, 
so widely applicable among the insects, will also apply. The only re- 
maining point of interest in this connection is the fact that the accessory 
disappears witli the ordinary chromosomes into the nuclear reticulum 
of the resting spermatid (figs. 60 and 61). In later stages the nueleus 
contains a conspicuous central plasmosome, but all trace of the accessory 
chromosome seems henceforth lost. 
The accessory chromosome thus in the opossum seems to appear 
slightly later and to disappear slightly earlier at the beginning and the 
end of spermatogenesis respectively, than in many of the insects (e. g. 
orthoptera). 
The occasional appearance of syncytial masses of spermatids (in- 
cluding 4 to 8 nuclei) such as Guyer (1909b) described also for the domestic 
chicken, may be noted. Probably no importance attaches to these pro- 
ducts of atypical development. Due to their rarity here their origin or 
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