ON THE STRUCTURE OF THE EGG IN THE BLOW-FLY. 683 
transfer of the micropyle from the posterior to the anterior 
egg-pole. 
The Vitelline Membrane is a structureless membrane in imme- 
diate contact with the yelk. Although many writers have 
even questioned the existence of this membrane, there cannot 
be the slightest doubt of its existence in the mature ovarian 
eggs of the Blow-fly. In the earlier stages of development its 
presence cannot be demonstrated. Korschelt says, ‘A vitelline 
membrane is present in Musca vomitoria, and in species of 
Vanessa, but in the former insect its first appearance is some- 
what late, later than the first rudiments of the chorion.’ 
My sections afford indications that the vitelline membrane is 
connected with the chorion at the margin of the micropyle, and 
Leydig [822] describes the union of the vitelline membrane and 
chorion in this region. 
The Yelk—When the freshly laid egg of the Blow-fly is 
crushed on a glass slide, and the yelk is examined, it is found 
to consist of a great number of oil globules less than ‘or mm. 
in diameter and of minute granules about 1 » in diameter sus- 
pended in a clear fluid. 
Sections made from ovaries imbedded in paraffin show that 
the yelk is shrivelled and irregular in outline; this is probably 
the result of the solution of the fat, as no fat granules are 
found in the yelk, although some preparations exhibit vacuoles 
in their place. The granules stain readily with logwood and 
carmine, and are probably proteid granules. In such sections 
the granular yelk is seen surrounded by a clear zone in which 
both granules and fat droplets are absent. This layer is 
present in the eggs which lie in the ovarian follicles. It was 
observed by Weismann in impregnated eggs, and he termed 
it ‘blastoderm plasma,’ and believed that the blastoderm is 
developed from it. Although I have prepared vast numbers 
of serial sections of ovaries filled with ripe eggs, and examined 
them most carefully section by section again and again, I have 
never found any structures which resemble a germinal vesicle 
or spot. I believe, however, that leucocyte corpuscles similar 
to those of the blood of the mother are recognisable in some 
