402 F. M. BALFOUR. 



Dr. Alex. Shultz^ has more recently investigated a considerable 

 variety of genera and finds three conditions of the egg mem- 

 branes. (1.) In Torpedo, a homogeneous membrane, which is 

 of the nature of a chorion. (2.) In Raja, a homogeneous 

 membrane which is, however, perforated. (3). In Squalid^, a 

 thick homogeneous membrane, internal to which is a thinner 

 perforated membrane. He apparently regards the perforated 

 inner membrane as a specialised part of the simple membrane 

 found in Torpedo, and states that this membrane is of the nature 

 of a chorion. 



My own investigations have led me to the conclusion that 

 though the egg-membranes can probably be reduced to single 

 type for Elasmobranchs, yet that tliey vary with the stage of 

 development of the ovum. Scyllium (stcllare and canicula) and 

 Eaja have formed the objects of my investigation. I commence 

 with the two former. 



It has already been stated that in ScylHum, even before the 

 follicular epithelium becomes formed, a delicate membrane 

 round 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 Gegeubaur, 

 and homogeneous membrane of Schultz. 



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

 0'12 mm., this membrane, though considerably thicker than in 

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

 ova of 05 mm. from a young female (PL XA^III, fig, 21) the 

 vitelhne membrane has a thickness of 0-002 mm. and is quite 

 homogeneous.- Internally to it may be observed very faint in- 

 dications of the differentiation of the outermost layer of the vitellus 

 into the perforated or radially striated membrane of Schultz, 

 which will be s])oken of as zona radiata. 



In an ovum of 1 mm. from the nearly full grown though not 

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

 ness and deliniteness, and may measure as much as 000 i 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 1 mm. in diameter, both vitelline membrane and 

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



1 "Zur Entwicklungsgeschichte d. Selacliicr," 'Arch. f. niikr. Anat.,' 

 vol. xi. 



2 Tlie apparent structure in the vitelline membrane in my fiu'nre is merely 

 intended to rcjjresent tlie dark colour assumed by it on being stained. The 

 Zona radiata has been n)ade rather luo thick by the artist. 



