136 BULLETIN OF THE 



this region ; it probably is entirely wanting in the area immediately sur- 

 rounding the micropyle. The granulosa cells are two or three layers 

 deep in the vicinity of the micropyle, and a single cell larger than the 

 others is always to be found directly above the canal. It usually sends 

 a prolongation into the canal itself (Figs. 18, 21). 



In eggs about to be laid, the greatest thickness of the zona in the 

 vicinity of the micropyle is approximately 24 jx, and the thickening in 

 this region is not so conspicuous as at the earlier stage. The zona bends 

 inward slightly, so that its inner surface no longer forms a simple 

 curve. The micropylar passage through the zona presents three re- 

 gions : a shallow funnel-shaped depression, which occupies the outer 

 third of the layer ; a narrower tubular portion, which is a prolonga- 

 tion of the bottom of the funnel, and is rounded at its lower end ; 

 and finally a very narrow canal, which traverses the inner sixth or 

 eighth of the zona, and opens at the apex of the low elevation of the 

 inner surface (Fig. 18). 



The outer or funnel-shaped portion is wholly filled even at this 

 advanced stage by the single large micropylar cell which was seen at 

 the earlier stage (Figs. 18, 21). 



Perca. 



The egg of the perch has been a favorite subject for study. Almost 

 every writer on teleostean ova has examined it. Von Baer ('35, pp. 6, 7) 

 first described it as having a double membrane, the outer portion being 

 traversed by long narrow dark spots (" dunklern Flecken "). 



Muller ('54) gives a fuller account. He separates the membrane into 

 an inner, the zona radiata, and an outer, the capsule. The outer sur- 

 face of the zona is described as being covered with exceedingly small 

 cylindrical projections. These are doubtless nothing but the elevations 

 between the pore-canals, which are rather wide on the outer half of the 

 zona. The capsule is radially traversed by small spiral tubes, which 

 are enlarged and funnel-shaped at both ends. Transverse filaments are 

 sometimes seen between these radial tubes. On applying pressure, yolk 

 granules were forced into the spiral tubes, but in no case was any yolk 

 matter forced between the tubules ; from which he concludes that the 

 capsule must be closed between them. 



Kolliker ('58) discusses the origin of the " tubules." He considers 

 them to be outgrowths from the follicular cells, and the substance between 

 them as a secretion from those cells. He denies the statement made by 

 Muller, that they are hollow, but has seen the anastomosing filaments 



