920 THE AMERICAN NATURALIST. [Vor. XXXV. 
follicles (2) the apical third is occupied exclusively by sperma- 
togonia, some of which can be seen in mitosis ; the basal region, 
on the other hand, by spermatids in an immature state. Fol- 
licle cells occur on the outside of the follicle, being especially 
abundant in the basal region. In the adjacent follicle (4) the 
apical two-thirds is occupied by cells in the synapsis stage, 
the remaining portion by spermatids in a stage of development 
later than that of follicle (2). Along the margin of the follicle 
are found scattered small groups of spermatogonia (sg). The 
third and most anterior follicle (c) contains chiefly spermato- 
cytes in a late prophase. Groups of spermatogonia similar to 
those of follicle (^) are here also found scattered along the 
margin and nearly filling the extreme apical portion. The 
follicle cells in the basal region are undergoing not only active 
amitotic division, but to a certain extent degeneration. Their 
active multiplication or fragmentation causes them to crowd in 
towards the axis of the follicle. 
From a comparison of the extent of these growth regions in 
the three follicles, the developmental cycle may be conceived 
somewhat as follows. The spermatozoa, when fully formed, 
are forced into the vas. Since they have no motion of their 
own, this is probably caused by the contraction of the muscle 
layer of the follicle, perhaps assisted by the pressure of the 
growing cells in the apical region. During this process the 
spermatogonia in the apical portion of the follicle divide and 
come to fill up the space left vacant by the discharged sperm. 
The majority of the spermatogonia thus filling up the follicle 
proceed in their development, while the remainder form the 
groups of cells along the margin of the follicle already described 
in follicles (4) and (c), and which are destined to supply a new 
generation of cells. The spermatids also proceed in develop- 
ment and are forced into the vas. 
A condition like that represented in follicle (4) thus arises, 
the basal region filled with spermatozoa in a late stage about 
to pass into the vas deferens, and the apical region with cells 
which have progressed as far as the synapsis stage. Later, the 
spermatozoa having been completely discharged, the cells of 
the apical region come to occupy the basal part of the follicle, 
