DAVIS: SPERMATOGENESIS. 145. 
Platner (89), Wilcox (95) Erlanger (97), Baumgartner (:02) and 
Munson (:06) have maintained that it is formed almost entirely from 
the interzonal filaments. In Dissosteira I think there can be no: 
question that the Nebenkern is not formed, directly at least, from the 
interzonal filaments, since they can be plainly distinguished after the 
formation of the Nebenkern is well advanced. It is of course possible 
that as the interzonal filaments disappear the material derived from 
their disintegration may aid in the formation of the Nebenkern, but 
this seems scarcely probable. On the other hand there is good evi- 
dence that it is formed from the mitochondrion. 
V. SUMMARY. 
1. In all the forms studied there is a single apical cell of charac- 
teristic appearance at the distal end of each follicle. 
2. ‘The primary spermatogonia surround, and are in contact with, 
the apical cell. 
3. ‘The secondary spermatogonia are inclosed within a membrane 
formed by connective-tissue cells, the whole constituting a sperma- 
tocyst. 
4. All the spermatogonia in each cyst are the direct descendants 
of a single primary spermatogonium, which became surrounded by 
one or more connective-tissue cells, and are, with rare exceptions, in 
practically the same stage of development. 
5. ‘The resting nuclei of the spermatogonia are irregular in shape 
and show a marked depression on the side adjacent to the greatest 
amount of cytoplasm. 
6. ‘The autosomes of the spermatogonia vary greatly in size, and 
can be readily arranged in symmetrical pairs, which usually lie close 
together, and show constant and characteristic differences in both 
form and size. 
7. A monosome is always present. 
8. ‘The oégonia contain one more chromosome than the sperma- 
togonia, there being a symmetrical pair in place of the monosome. 
9. In the resting spermatogonia of Steiroxys trilineata the mono- 
some is inclosed within a separate vesicle. 
10. During the telophase of the last spermatogonial division the 
monosome, which is inclosed within a distinct vesicle, retains its com- 
pact form and often shows a more or less distinct bipartite structure. 
11. The first stage of the primary spermatocyte is characterized 
by the chromatin being evenly distributed through the nucleus in a 
finely granular condition. 
