140 BULLETIN OF THE 



cells stain more deeply than the inter-tubular substance of the capsular 

 layer. This peculiarity is very serviceable when one is searching for 

 the micropyle. Notwithstanding the absence of a broad depression, 

 there is a narrow irregular canal left in the centre between the modified 

 granulosa cells, which can best be seen upon sections tangential to this 

 part of the egg. (Figs. 27-32, Plate II. Compare Explanation of Fig- 

 ures.) The appearance is similar to what one might imagine would 

 result if the central cell of this region had dropped out of its original 

 place. That such a cell has not wholly disappeared, but has simply 

 lost its peripheral connection with the wall of the theca, is rendered 

 probable by the presence of a peculiar cell at the bottom of this canal. 

 Directly over the micropyle, in contact with the zona and filling more 

 or less completely its micropylar depression, lies a single cell of large 

 size. Its nucleus is more nearly spherical than the nuclei of the other 

 cells, and it is not stained as deeply as they are. (Fig. 24, Plate III. ; 

 and Figs. 26, 31, Plate II.) There can be no doubt that it is a 

 peculiarly modified granulosa cell. 



Morone americana. 



The egg membrane of the white perch has never been described, but 

 Ryder ('82) has described the micropyle. 



There is only a single membrane, the zona radiata, but it is composed 

 of two distinct layers, both of which are traversed by pore canals. The 

 eggs examined were taken from fishes caught in February, April, and 

 May. In February the ovary contained eggs in four stages of develop- 

 ment ; in the older stages there are well developed membranes. Eggs 

 of 0.16 mm. in diameter have a single homogeneous membrane 1.2 p 

 thick. When they have reached a diameter of 0.28 mm. the zona is 

 composed of two layers (Fig. 33, Plate II.), a very thin inner and a 

 thicker outer one ; together they measure 39 p in thickness. By the 

 time the eggs have reached a diameter of 0.10 mm. (Fig. 34, Plate II.) 

 the total thickness of the membrane is more than doubled ; that of the 

 outer layer is 49 /x and that of the inner 39 /x. The outer layer is 

 formed first and takes a deeper stain. It does not increase much in 

 thickness after the appearance of the inner layer, and in the older eggs 

 it contains vacuoles. The inner is at first apparently homogeneous, but 

 with its great increase in thickness there appear in it the radial stria- 

 tions characteristic of the zona. The granulosa cells are small and low, 

 and have flattened nuclei situated in the middle of the cell. 



