MUSEUM OF COMPARATIVE ZOOLOGY. 45 



the shell. In this deflection of the membranes, the zona radiata seems 

 to bend more abruptly than the villous layer ; this, however, is due 

 principally to the fact that the region of greatest curvature is also the 

 region of most rapid change in the thickness of the zona. From this it 

 results that the inner contour of the zona is much more abruptly curved 

 than the outer, in some cases appearing almost angular. As a further 

 consequence of this, the conical elevation appears to arise abruptly from 

 the inner surface of the membrane ; its apex is rounded, and in the 

 ovarian egg its surface is everywhere in contact with the yolk. An 

 inquiry as to whether this infolding is the result of a process of absorp- 

 tion, or is due to a peculiar local modification of the activities which 

 produce the membrane, will best be deferred until I have given a de- 

 scription of the layer of cells which immediately invests the ovarian 

 ovum. 



The micropyle proper, or the micropylar canal (Plate I. Fig. ll a , 

 Plate IV. Fig. 1, Plate VI. Figs. 3, 4), is straight and of uniform calibre. 

 It begins at the centre of the bottom of the funnel, and passes through 

 both villous and zonal layers of the egg membrane ; it is about 8 fx long. 

 Its cross section is circular and about 2 fx in diameter. There is no flare 

 to the canal, either at the external or internal end, so far as I have been 

 able to observe. I am unable to say whether the diameter which I have 

 given is that which the micropyle possesses at the moment the egg is 

 laid. From measurements of spermatozoa allowed to dry upon the slide 

 (Plate VII. Fig. 3), one would imagine that the calibre of the micro- 

 pylar canal must be at least 3 ll, that being the diameter of the heads of 

 spermatozoa thus treated ; but according to measurements made upon 

 living spermatic cells the heads are only about 1.8 /x in diameter, so 

 that I think 2 /x is probably the normal average calibre of the canal. 

 Still, I have sections in which its diameter is 2.5 /x, and in the case of 

 some fresh membranes it was only 1.5 fx in diameter. The narrowness 

 in the latter case I attribute to the swelling of the zona when exposed 

 to water and glycerine, in which the membranes were examined. 



c. Granulosa. 



Nearly mature ovarian eggs are closely enveloped by an uninterrupted 

 cell layer, which is everywhere in contact with the outer surface of the 

 villi. Over the greater part of the egg this layer — the follicular epithe- 

 lium or granulosa — is composed of thin, flat polygonal cells, arranged in 

 a sheet only one cell thick. In surface views the granulosa cells (Plate V. 

 Fig. 4) appear of fairly uniform size, — 15-20 /x in diameter, — are 



