i[3] EMBRYOGRAPHY OF OSSEOUS FISHES. 457 



general survey of the ground which I have traversed, until quite recently. 

 The observations in addition to those on the development of the cod 

 were made principally at Cherrystone and New Point Comfort, Va., 

 Havre de Grace, Md., and Washington, D. C. 



2. — The ova and ovaries of the cod and other fishes. 



The mature eggs of the cod measure 1.3 millimeters in diameter, or 

 one-nineteenth of an inch, and are covered by a vitelline membrane which 

 is not porous or enveloped in adhesive material.* It is thin, A^ery trans- 

 parent, and laminated, as has been stated by Sars, and at one point is 

 perforated by a single minute opening, the micropyle; the membrane is 

 somewhat thicker in the immediate vicinity of this opening. In figures 

 1, 6, and 7, the micropyle is shown at the lower i)ole of the egg, and in 

 figure 5, very much magnified, a portion of the surrounding membrane, 

 together with the form of the tube of the micropyle, is seen in optic 

 section. The outer opening is situated in a funnel-shaped depression, 

 the rim of which is defined from the surface of the membrane by a fur- 

 row running round it. From the funnel-shaped outer opening a fine 

 canal passes inwards to end in the center of another wider funnel- 

 shaped depression dn the inner surface of the membrane at mi', but which 

 is situated upon a considerable internal elevation. As far as the writer 

 has been able to make out with very excellent lenses, this is the only 

 opening into the cod's egg through which communication is established 

 between the water surrounding it and the space inside between the vi- 

 tellus and the vitelline membrane. 



In other si)ecies the character of the egg membrane is quite differ- 

 ent, since it is often found that the whole surface of the egg membrane 

 is very regularly perforated by very fine canals, so that when it is viewed 

 in optic section under the microscope numerous fine radiating striae are 

 found traversing the membrane in a direction vertical to its external 

 surface. These striae are due to the presence of fine canals, which may 

 open at the apices of minute papillae, as we find in the case of the mem- 

 brane of the shad's egg ; in other cases we may find the surface marked 

 iis if by fine lines crossing each other at definite angles. An egg mem- 

 brane which is minutely perforated, as above described, is known as a 

 zona radiata, a name proposed by Waldeyer. 



The cod's egg is without the zona radiata found inclosing the egg 

 proper of the shad, wliitefisli, and sculpin, and, inasmuch as it is un- 

 questionably true that a micro])yle perforates the zona in a number of 

 these cases, it does not appear that sufhcient grounds exist for the dec- 

 laration that a micropyle perforates the zona radiata alone, in the face 

 of the fact that the vitelline membrane only is perforated in this one 

 instance. 



Waldeyer holds that the vitelline membrane is a secretion from 

 the cells of the follicle in which the ovum is developed. Lereboul- 

 let regarded it as a chorion, a conception of it which has now been gen- 



