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 



