SPERMATOGENESIS OF EUSCHISTUS 791 
togenetic relations of most insects, a true idiozome arises in the 
spermatocytes, mitochondria develop around it, the idiozome 
disintegrates and a sphere appears, thissphere disappears entirely 
and in the spermatid another sphere is produced that originates 
another new body, the perforatorium. 
It is clear that the first spermatocytes and oocytes are the 
most interesting cells of their respective cycles, for they exhibit 
the most significant processes—conjugation of chromosomes, 
reduction division, elaboration of mitochondria. In them the 
history of the cytoplasmic parts is markedly epigenetic. And 
another series of epigenetic changes is exhibited by the histo- 
genesis of the spermatozoon. 
There is then a parallel with the somatic cycle, which begins 
with an undifferentiated and terminates with a highly differen- 
tiated condition. The ripe ovum and spermatozoon are much 
more differentiated than the spermatogonium or oogonium; 
each of them enters, with the commencement of the growth 
period, upon its period of specialization. 
The recognition of this resemblance may throw light upon 
the problem of the segregation of the germ cells. By what 
process is it that certain cells of the embryo are held back from 
somatic differentiation to become germ cells? In other words, 
why do not all the cells become specialized? The answer, it 
seems to me, is to be sought in the distribution of the specializa- 
tions of the fertilized egg to the cells of the embryo. One set 
of specialized structures of the germ cells, the mitochondria, 
are now known to give rise to various important specializations 
of body cells. The mitochondria, that are elaborated, in greatest — 
part at least, during the growth period of the germ cells, persist 
from the fertilized egg into cleavage stages, and ultimately 
transform into various specialized fibrillar structures. With 
this in mind the setting aside of germ cells from body cells 
could be explained mechanically as follows: any cleavage cell 
which failed to receive mitochondria, or failed to receive particu- 
lar ones or a particular amount of them, would be incapacitated 
from engendering such somatic specializations, it would thereby 
become a germ cell. This might appear contrary to the idea 
