MUSEUM OF COMPARATIVE ZOOLOGY. 43 



b. Micropyle. 



The micropyle was apparently overlooked by Balfour and Parker, 

 since it is not mentioned by them ; nor has it been mentioned, I believe, 

 by any one else, although it occupies a region which is so conspicuously 

 marked that, having once seen it, one could readily find it with the aid 

 of a simple lens. Except in eggs that have lain for some time in water, 

 the region of the micropyle appears, when seen under a hand lens, like 

 a minute hole in the shell ; in surface views with a higher power it looks 

 like a deep circular pit (Plate IV. Figs. 3, 4) sunk in the egg membrane. 

 Its diameter is five or six hundredths of a millimeter. Its outline is 

 nearly always circular, and it has a clearly cut edge. In a few cases a 

 cross section of the pit has proved to be oval instead of circular, occasion- 

 ally with one diameter of the oval more than twice as long as the other 

 (Plate VII. Fig. 4). A similar appearance, though not so marked, is often 

 produced, even when the pit is really circular, if the plane of the section 

 is oblique to its axis. Sometimes the pit is partly filled by a whitish, ap- 

 parently spheroidal body (Plate IV. Fig. 4). When the egg is so viewed 

 that this depression lies in the equator, the profile of the egg in its vicinity 

 may be slightly modified, and show a low conical elevation, at the apex of 

 which the pit is located. This is not commonly the case, however, for 

 usually there is nothing in the profile to denote the position of the pit. 

 In eggs nearly mature, and in those which have been recently laid, its 

 place can be easily found by its relation to the lighter colored animal 

 pole of the egg. It is invariably located over some part of the germinal 

 area, and usually precisely over its centre (Plate IV. Fig. 3). 



The real nature of this pit and its relations to the two layers of the 

 egg membrane and to the yolk can be studied on optical, but still better 

 on actual sections. For a general survey radial sections are most in- 

 structive, but for the elucidation of some questions sections tangential 

 to the egg at the animal pole are more valuable. 



In strictly radial sections through the region of the micropyle, it is 

 to be seen that the surface of the egg is deeply depressed. The form of 

 the depression varies somewhat in different eggs, from that of a funnel, 

 i.e. with sloping walls (Plate IV. Figs. 1, 5), to that in which the walls 

 are for some distance almost parallel (Plate V. Fig. 2). This depression 

 results from an infolding of both layers of the egg membrane ; it forms, 

 however, only an approach to the true micropyle, or micropylar canal, 

 the latter being a minute passage through both layers which begins at 

 the bottom of the depression. 



