The Spermatogenesis of Hydra. 395 
cleus next changes its shape and becomes a flattened spheroid; the 
centrosomes appear over the poles. The spiral thread of chromo- 
meres is no longer a spiral, but the halves of each turn have come 
to lie meridionally with reference to the poles, each half containing 
four chromomeres. The thread now divides at the poles, forming 
twelve meridional segments. 
Equatorial plate. 
Each segment contracts, the chromomeres fuse more or less 
completely and twelve approximately spherical chromosomes are formed 
in an equatorial position (Fig. 36). 
Metaphase. Anaphase. Telephase; reduction in 
number of chromosomes. 
An interesting feature of the spermatogenesis of Hydra is the 
frequent distinctness and persistence of these chromomeres. Fig. 37 
shows a nearly spherical spermatogonium in the metaphase. Two 
chromomeres are to be made out in most of the daughter chromo- 
somes. The karyokinetic figure is simple. A single fibre runs from 
each chromosome to the centrosome. Sometimes polar radiations 
appear; when they do they seem like continuations of the spindle 
fibres through the centrosome to the cell wall. In the spermato- 
cyte division traces of central spindle fibres are to be found. They 
have not been noticed in the spermatogonic divisions. As the chromo- 
somes pull apart they are connected by interzonal fibres and we 
often get, especially in a region where the cells are not very crowded, 
such a typical division figure as 39 where the ‚Zwischenkörper‘ 
are visible. As the twelve daughter chromosomes approach the 
poles in the separating daughter cells there is visible a tendency 
toward reduction of their number by the fusion of adjacent chromo- 
somes. Fig. 40 shows this process plainly. Usually this process is 
delayed until the chromosomes are densely massed in the new 
daughter nucleus. In the upper nucleus of Fig. 38 nearly all trace 
of the individual chromosomes is lost. In the lower nucleus the 
chromosomes are still apparent and fusion of the adjacent ones is 
progressing. This fusion begins apparently at the tips nearest the 
poles which are naturally approximating as the chromosomes con- 
verge and there is temporarily produced a somewhat V-shaped 
figure. 
Zool. Jahrb. XXI. Abt. f. Anat. 26 
