144 BULLETIN OF THE 



the middle of the attached side of the egg. The direction of the inner 

 surface of the zona was not altered in the vicinity of the micropyle 

 (Fig. 38, Plate II.), but its outer surface exhibited a broad circular 

 depression, by which the thickness of the zona was diminished about one 

 half. The micropyle proper at the centre of the depression appeared 

 as a narrow canal of uniform calibre. Between the zona and the yolk 

 there was a narrow space, probably formed by the contraction of the 

 yolk ; beneath the micropylar region, this space was abruptly enlarged 

 into a hemispherical depression. Across this space the radial strands of 

 protoplasm characteristic of almost all the spaces between the zona and 

 the yolk were plainly visible. 



The granulosa, which over all other parts of the egg is composed, as 

 usual, of a single layer of cells, is thickened in the region of the micro- 

 pyle. As the direction of the long axes of their oval nuclei show, the 

 cells near the margin of the micropylar depression in the zona have 

 their peripheral ends inclined toward the axis of the micropyle. The 

 cells which fill up the depression have larger and more elongated nuclei, 

 and the obliquity of the latter has become so great that, the depression 

 appears to be filled with a granulosa layer two or three cells deep. It 

 would seem that in this case the single micropylar cell found in other 

 eggs was represented by a number of enlarged granulosa cells. 



Clupea vemalis. 



The chief interest in the egg membranes of this species centres in the 

 presence of a thin, highly refractive, structureless membrane overlying 

 the zona radiata of eggs in an advanced stage of development (Fig. 39, 

 Plate III.). This outer membrane is intimately connected with the gran- 

 ulosa cells, so that it usually retains its connection with the granulosa 

 when the latter is artificially separated from the zona. In all such cases 

 slender striations extend from it to the zona radiata. The appearance of 

 these markings is such as to show clearly that they are prolongations of 

 the substance of the outer membrane, and there can be little doubt that 

 the projections penetrate the pore canals of the zona radiata, from which 

 they are partially withdrawn by the artificial separation of the two mem- 

 branes. This structural condition suggests an explanation for simi- 

 lar appearances beloiv the zona radiata in Esox and other fishes, and 

 between the zona radiata and an inner layer in Amiurus (Fig. 45) and 

 Ictalurus. It will be more fully discussed later. 



