OF TIIK VKRTEHKATK OVAI<\. 571 



the ovum can be demonstrated, which appears to me to be 

 derived from the vitellus or body of the ovum, and is therefore of 

 the nature of a vitelline membrane. It becomes the vitelline 

 membrane of Leydig, the albuminous membrane of Gegenbaur, 

 and homogeneous membrane of Schultz. 



In a young fish (not long hatched) with ova of not more than 

 O'I2 mm., this membrane, though considerably thicker than in 

 the embryo, is not thick enough to be accurately measured. In 

 ova of o - 5 mm. from a young female (PI. 25, fig. 21) the vitelline 

 membrane has a thickness of O'OO2 mm. and is quite -homo- 

 geneous 1 . Internally to it may be observed very faint indications 

 of the differentiation of the outermost layer of the vitellus into 

 the perforated or radially striated membrane of Schultz, which 

 will be spoken of as zona radiata. 



In an ovum of I mm. from the nearly full grown though not 

 sexually mature female, the zona radiata has increased in thick- 

 ness and definiteness, and may measure as much as 0*004 mm. 

 It is always very sharply separated from the vitelline membrane, 

 but appears to be more or less continuous on its inner border 

 with the body of the ovum, at the expense of which it no doubt 

 grows in thickness. 



In ova above I mm. in diameter, both vitelline membrane and 

 zona radiata, but especially the latter, increase in thickness. 

 The zona becomes marked off" from the yolk, and its radial striae 

 become easy to see even with comparatively low powers. In 

 many specimens it appears to be formed of a number of small 

 columns, as described by Gegenbaur and others. The stage of 

 about the greatest development of both the vitelline membrane 

 and zona radiata is represented on PI. 25, fig. 22. 



At this time the vitelline membrane appears frequently to 

 exhibit a distinct stratification, dividing it into two or more suc- 

 cessive layers. It is not, however, acted on in the same manner 

 by all reagents, and with absolute alcohol appears at times longi- 

 tudinally striated. 



From this stage onwards, both vitelline membrane and zona 

 gradually atrophy, simultaneously with a series of remarkable 



1 The apparent structure in the vitelline membrane in my figure is merely in- 

 tended to represent the dark colour assumed by it on being stained. The zona 

 radiata has been made rather too thick by the artist. 



372 



