ON THE STRUCTURE OF THE EGG IN THE BLOW-FLY. 681 
whether this cuticle is an exudation from their inner surface, is 
a question similar to that involved in the origin of the cuticular 
structures of the integument, which has already been discussed. 
Leydig, who believes that all cuticular structures are formed as 
exudations from the surfaces of cells, naturally regards the 
chorion as having a similar origin, and the hexagonal fields as 
imprints of the cells. As I have no doubt that cuticular mem- 
branes are formed by the direct metamorphosis of the surface 
layers of the cell protoplasm, I regard the chorion as the result 
of such a metamorphosis. 
In the Blow-fly I believe the whole thickness of the cellular 
Fic. 97.—Details of the structure of the chorion. A, a transverse section through 
the micropyle canal, near the equator of the egg; m, micropyle canal, +'y oil, 
immersion ; &, a surface view of the micropyle ; c, rosette of cells, 4s oil immer- 
sion ; C, a surface view of the chorion showing the punctate hexagonal fields, 
4 objective; , a diagrammatic section of the anterior pole of the chorion; 
c, outer cellular layer of the chorion ; c#, cuticular layer of the chorion ; 4, hood 
of the micropyle ; c/, cleft in the hood continuous with the micropyle canal ; 
m, micropyle ; 7c, micropyle canal seen through the semitransparent chorion. 
layer is transformed into cuticular tissue. In those insects in 
which the epithelial layer of the chorion is shed as a ‘corpus 
luteum’ in the oviduct or left in the egg-tube, it appears 
probable that only a part of the outer epithelial layer is cuticu- 
larised, or the chorion may consist of the inner layer of cells 
only in its final stage of development. 
The Micropyle is a minute opening at the anterior pole of the 
egg, variable in form, sometimes quadrilateral and sometimes 
circular, of from 2°5 » to 5 » in diameter, surrounded by a rosette 
of epithelial cells (Fig. 97). I believe the variations in size 
and form which I have observed are due to the contraction of 
