694 THE GENERATIVE ORGANS. 



cells, owing to the great transparency of its embryos. Lee's 

 observations conform with those of Hertwig, who says : 



' When the cavity of the Gastrula (in Sagitta) begins to 

 enlarge, two cells appear in the endoderm, which touch each 

 other, and which are situated at the aboral pole of the blasto- 

 derm. These cells are conspicuous owing to their size, as 

 they are much larger than the ordinary embryonic cells at 

 this epoch ; but what renders them still more remarkable is 

 their large vesicular nuclei and numerous nucleoli. These are 

 the primordial sexual cells.' 



I have already referred to the observations of Balbiani, who 

 considers that the sexual cells in the Muscidae originate from 

 the polar cells of Weismann ; and if, as has been maintained 

 here, these cells represent the primitive endoblast, derived 

 from the complete segmentation of a germ yelk, the origin of 

 the primitive sexual cells in the Muscidae would correspond 

 with that of the same cells in the Chaetognatha. It must be 

 remembered that there are no direct observations that can be 

 considered quite convincing on the subject. I think, however, 

 there is much evidence in favour of the view that the primitive 

 sexual cells in Insects originate from the primitive endoderm 

 of a segmenting morula, since, as has been already stated I 

 regard the polar cells as part, at least, of the vegetative pole 

 of this morula. 



Polyblasts are large multinucleated cells, which are seen in 

 groups lying next the spermatospores and their encapsulated 

 daughter-cells. They are apparently formed by the rapid 

 growth of the daughter-cells, accompanied by very active 

 divisions of the nucleus. The polyblasts are from 15 /A to 25 //, 



DKsCKirilON (>! I'l.ATK XI, VI 1 1. 



of the testis of the mature male Hlow-lly. .-/, a section of the testis and sur- 

 rounding fat body, seen with a \ inch objective. />', a portion of a similar pre- 

 paration, seen with a ,'._. oil immersion objective. The fat-cells are not fi^uiei 1 . 

 ( ', immature spermatozoa, seen with a ,'.. immersion lens: a, cellular capsule of 

 the testis ; /<, nucleated cell, probably trabecular tissue dividing the testis into 

 loculi ; f, capsule enclosing spermatic filaments; in, capsule of the testis; 

 /, nolybla-t cells ; j, j, spermatospores ; s J\ spermatozoa. 



