The Sperma togenesis of tlie Opossum (Didelphys virginiana) etc. 77 
5 ) In primary spermatocytes of tlie white mouse there is present in the nucleus 
during the stage intervening between synizesis and the prophase of the first maturation 
division (i. e. from the synaptenic through the pachytenic phase) a body which behaves 
in all respects like a heterochromosome. It is an oval (sometimes somewhat irregulär, 
i. e. bipartite and cone-shaped) sharply contoured body, close to the nuclear membrane, 
and usually at the point where the idiozome lies. Sometimes it is clearly in close Con- 
nection with the segmenting spireme. Similar appearances present themselves in los 
taurus (Schoenfeld shows this monosome-like body, “corpuscule intra-nucleaire”, in 
his figs. 4, 26 and 30) and the horse. In the latter the auxocytes are considerably smaller 
than in the bull and thus more difficult to study. The heterochromosome in the horse 
is small and frequently appears double or even triple. Nor is it as definitely and in- 
variably oriented as in the opossum. Due to the large number and the crowding of 
the chromosomes it cannot be followed through the mitoses. The conspicuous pre- 
ssnce of the accessory chromosome in the horse is apparently very transient. AU of 
these cases are under further investigation. At present I am unable to elucidate the question 
either of the origin or fate of this body in the above three cases. On the basis of its 
appearance in early prophase it may tentatively be interpreted as an accessory chromo- 
some. Neither in the resting primary spermatocytes nor in the maturation mitoses 
can it be definitely identified. It would seem that a chromosome (probably bivalent) 
of the ordinary type here for a brief period b haves like a heterochromosome, thus 
only revealing its true nature as a specific element possibly with a specific function, 
whatever this may be. It seems probable that heterochromosomes of various dcgrees 
of differentiation wiU be discovered in aU forms, their apparent absence being due to 
our inability to recognize them. In the squirrel, for example, I am unable to recognize 
any chromosome at any stage comparable to a heterochromosome ■ — and yet if one 
is essential to the opossum its analogue would seem to be equally essential to the squirrel. 
If one is present in the squirrel it certainly is at no time polarized and must be very 
obscure or very minute. Van Molle’s (La CeUule 23 and 24, 2) Ulustrations of early 
auxocytes of the squirrel also do not show any structure resembling an accessory chromo- 
some; though “chromoplastes” appear in the leptotenic phase. 
6 ) Nuclei in the synizesis phase appear abundantly in the smear preparations 
described in foot-note 3. Careful comparison with nuclei at the same stage in 
sections reveals practicaUy no difference in consequence of fixation. 
7 ) Except as the paired mitochondria above noted may bear the Interpretation 
suggested. 
8 ) More recently Meves (Arch. f. mikr. Anat. u. Entwickl. 16 : 4) bas used the 
terms “plastomes”, “plastoconts” and “plastochondria” in a general sense. Schaxel 
(Arch. f. mikr. Anat. u. Entwickl. 16 : 3) uses the term “kinetocliromidia” for these 
same bodies. 
9 ) It may be that chromidia are the forerunners of botlr the tigroid substance 
and mitochondria, the latter further differentiating into neurofibrillae. 
10 ) According to Schoenfeld’s figs. 1 to 5, the sphere (idiozome and Neben- 
kern) has a history in los taurus very similar to what is here described for the 
opossum and other mannuals. 
1 : ) In the squirrel. according to the illustrations of va.\ Molle, the homologues 
of the Nebenkern and idiozome of the opossum appear to liave a similar history. 
'The migrating portion of the original sphere (idiozome) disappears witliin the posterior 
cytoplasmic mass as the “sphere”, while the persisting anterior portion (Nebenkern) 
