Fig. 498.—Micropyle (Mk) of eggs; a, of a fly, Antomyia; b, Drosophila cellaris; c, stalked egg of Paniscus testaceus.—After Leuckart, from Perrier.
When the eggs are deposited in exposed places, and remain in such situations for several days, or weeks, or even through the winter, the shell is either solid and strengthened by the ribs and ridges; or the shell, if of winter eggs, is unornamented, and is dense and solid, to resist extremes in temperature or the attacks of egg-eating birds, mites, etc.
The micropyle.—This is an opening or canal, or, as in most insects, a group of canals situated at the anterior end of the egg for the entrance of the spermatozoa during the process of fertilization of the ovum (Fig. 498). In Acrydians, however, the micropyle is situated at the posterior end of the egg. The micropyle (Fig. 499) is a complicated apparatus within whose circumference the vitelline membrane appears to be firmly attached to the chorion, so that the perforation passes through the chorion as well as the yolk-membrane.
The micropyles of the cockroach are probably as simple and generalized as in any insect. Wheeler states that they are in Phyllodromia scattered over the end of the egg, “over a quadrant of the upper hemisphere, where the beautiful hexagonal pattern of the chorion gives away to an even trabeculation.” The micropyles are wide-mouthed, very oblique, funnel-shaped canals, perforating the chorion, the apertures of the funnels appearing under a low power as clear, oval spots, the long axis of which is parallel to the long axis of the egg.
Fig. 499.—a, fragment of a micropylar papilla, showing its lumen; b, optical section of another papilla, in this one the lumen extends to the vitelline membrane, but does not pass beyond it; c, d, e, and f, papillæ of different forms. A, anterior end of an ovarian egg, showing mode of growth of the micropylar papillæ: a, b, two successive stages; c, surface view of modified papillæ from the lower edges of the cap; d, tunica propria of the ovariole; e, remnant of the cell-mass that secreted (?) the micropylar cap.—After Ayers.
Fig. 500.—Egg of Perla maxima: c, chorion; d, oölemma; gs, glass-like covering of micropyle; l, cavity under same; g, canals penetrating chorion.—After Imhof, from Sharp.
“With a higher power the tube of each funnel is clearly visible as a thin canal which dilates rapidly into the large oval aperture on the outer face of the chorion. The narrow tube is sometimes fully as long as the large orifice. The micropylar perforations are all directed from the germarium to the vaginal pole of the egg. Hence a line, the hypothetical path of the spermatozoön, drawn through one of these oblique micropyles, and continued into the egg, would strike the equatorial plane. The female pronucleus, as we shall see further on, moves in this plane.” (Wheeler, p. 289.)
