782 THOS. H. MONTGOMERY, JR. 
to a true ‘Nebenkern’ (figs. 112, 113), lying at the distal end of 
the nucleus. This becomes spherical, stains more faintly than 
before, and differentiates into a central denser zone and a periph- 
eral (frequently vacuolated), more fluid layer (figs. 114-117). 
This mitochondrial Nebenkern then becomes irregularly pyri- 
form (figs. 118, 120), frequently with a concentric disposition of 
layers of different densities (fig. 119). Elongating still more 
it becomes divided into right and left halves (fig. 121), the axial 
thread lying between the two. One end of the now paired Neben- 
kern touches the nucleus near the centriole, while the other end 
grows backward into the tail of the sperm. Figs. 122 to 128 
illustrate further stages of the growth of this paired Nebenkern, 
showing how it keeps pace with the growth of the tail and how its 
two halves become spirally twisted upon each other. Up into 
the adult sperm (figs. 127-147) the mitochondrial substance of 
the Nebenkern is found enclosing the axial thread; but whether 
it extends the whole length of the tail in the adult sperm was not 
determined. It. becomes gradually more pale in appearance, 
perhaps due to its becoming thinner in mass. It is certain, how- 
ever, that this mitochondrial substance extends a long distance 
into the tail and is not limited to a small middle part; indeed, the 
adult sperm shows no part comparable with the well defined 
middle piece of mammalian sperm. 
E. Discussion 
In cells of the spermatogenetic cycle a number of larger struc- 
tures are found in the cytoplasm, though generally of less com- 
plicated nature than in egg cells because most sperm cells do not 
appear to produce yolk. 
One of these is the mitosome (Platner, ’89; Spindelrestkérper, 
Meves, 97) which arises in the telophase of mitosis from spindle 
fibers (connective fibers) persisting at the distal end of the cell. 
It is most prominent in spermatogonia, where it is a dense body, 
often continuing into a subsequent prophase, and often fusing 
with mitosomes of neighboring cells. In Euschistus a mitosome 
