THE PHDOGENETIC FLY, MIASTOR 67 
embryonic development; and (6) the contents of 
the primordial germ cell consist of the nucleus 
with undiminished chromatin and of all of the pole- 
plasm and apparently no other part of the egg sub- 
stance. 
The fact that only the primordial germ cell re- 
ceives a complete amount of chromatin is of particu- 
lar interest, since a similar condition has long been 
known in the case of Ascaris as we shall see later. 
It may also be noted in this place that the cyto- 
plasmic substance in the primordial germ cell may 
be recognized as the pole-plasm in the growing 
odcyte. Attempts have been made to determine the 
origin of this pole-plasm, but so far without success. 
It may be distinguished from the rest of the egg con- 
tents by its position at the posterior end and because 
of its affinity for certain dyes. It appears shortly 
before the maturation division is initiated, but no 
transition stages have been discovered — it has been 
either present or entirely absent in the preparations 
thus far studied. If we consider the history of this 
substance from the formation of the primordial 
germ cell to the growth period of the odcytes pro- 
duced by this primordial germ cell, we may conclude 
that at the time the multiplication period ends the 
pole-plasm has become equally distributed among the 
sixty-four odgonia. Then ensues the growth period 
during which the pole-plasm cannot be distinguished. 
Later, however, just before maturation, pole-plasm 
substance reappears which is equal in amount to 
that contained in the primordial germ cell of the 
