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 Muscide 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 Muscidz would correspond 
with that of the same cells in the Chetognatha. 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 uw to 25 
DEscrIPTION OF PLATE XLVIII. 
Sections of the testis of the mature male Blow-fly. 4, a section of the testis and sur- 
rounding fat body, seen with a } inch objective. 2, a portion of a similar pre- 
paration, seen with a 4"; oil immersion objective. The fat-cells are not figured. 
C, immature spermatozoa, seen with a ;'y immersion lens: a, cellular capsule of 
the testis; 2, nucleated cell, probably trabecular tissue dividing the testis into 
loculi; ¢, capsule enclosing spermatic filaments; #7, capsule of the testis; 
p, polyblast cells ; s, s, spermatospores ; s_/, spermatozoa. 
