DEVELOPMENT OF THE GENERATIVE ORGANS. 691 
think, of the view I have already suggested—that the so-called 
polar globules are really the segmentation spheres of the vege- 
tative pole of the egg. Their passage through the epiblast 
takes place, I conceive, by the blastopore; and I think it 
highly probable that some of these cells, or their descendants, 
are those from which the ovaries are developed. 
The number of polar cells described by Balbiani is eight. 
Other observers have seen sixteen, and some say twelve. The 
first gonad cells are only two in number, or, at most, four. 
Balbiani believes that the eight or more polar cells unite by 
fusion, and give rise to the primitive pair of sexual cells; this 
appears to me highly improbable, and it is supported by no 
direct evidence. It appears, however, that to a certain extent 
Balbiani’s statements are in consonance with my own obser- 
vations: he saw the entrance of the polar cells into the yelk 
by the blastopore, and traced the development of the gonads 
to a single pair of cells from the group of cells which I regard 
as the primitive hypoblast (p. 247). 
Brandt does not endorse the view that the primitive gonads 
originate from the polar cells, and rightly remarks that the 
resemblance of the first sexual cells to the polar cells in 
some insects cannot be regarded as evidence of their identity, 
and, further, ‘ that all embryonic cells are similar to the polar 
cells, as they have a similar origin.’ And I agree that all the 
hypoblastic elements, which originate from the polar cells if 
my views are correct, are precisely similar in appearance. I 
think that the observations of Balbiani undoubtedly show that 
the gonads are developed from the tissue which originates from 
the polar cells; but I do not regard them, as he apparently 
does, as the polar cells. I think, however, they, as well as all 
the hypoblastic tissues, originate from the polar cells. 
c. The Development of the Testes and their Ducts, and of the 
Paragonia in the Pupa of the Blow-fly. 
After the third day of the pupa the rudimentary testis is no 
longer reniform, but becomes pear-shaped and the genital 
