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 [322] 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 '01 mm. 

 in diameter and of minute granules about i fju 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 



