OVARIES liV TEf.EOSTEAN AND EL\SMOBBANCH PISHES. 181 



Origin of Egg Membranes. — This point has been the 

 subject of much discussion. The general consensus of opinion 

 seems to be in favour of the view that, with certain excep- 

 tions, such as the capsular layer in the perch, the egg mem- 

 branes are derived from the egg itself, being cuticle-like 

 secretion therefrom. Balfour maintained that the vitelline 

 membrane of Raia and Scy Ilium was no less than the zona 

 radiata the product of the egg"; that in some cases the 

 vitelline membrane was formed before the egg possessed a 

 distinct follicular epithelium. Schultze (1875), however, held 

 the opposite view. Van Beneden's observation (1880) on the 

 bnt seems to prove that the zona pellucida (zona radiata) is 

 or can be formed without the intervention of the follicular 

 epithelium. Van Beneden frequently observed that where 

 two or more ovarian ova were in contact over a considerable 

 area the zona pellucida was as thick at the places where the 

 eggs touched as at those regions where they were invested 

 with the follicular epithelium. 



Mark discusses exhaustively the origin of the egg mem- 

 branes in Lepidosteus. In this species there are two, an 

 outer and an inner, called by Mark the villous layer and zona 

 radiata respectively. The villous layer is the thicker, and is 

 composed of closely set villi, radially arranged with reference 

 to the centre of the egg. "Each granulosa cell corresponds 

 in number to four to eight villi, but there is no constancy in 

 the position of the cells or their nuclei in reference to the 

 underlying villi." From various evidence, Mark is "certain 

 that the layer which is first to make its appearance between 

 the yolk and the follicular epithelium is the villous layer ; " 

 and that, like the zona radiata, the villous layer of the egg 

 membranes in Lepidosteus is also the product of the ovum 

 itself rather than of the follicular epithelium surrounding it." 

 Further, Mark has clearly demonstrated the existence of pore 

 canals in the zona radiata of Lepidosteus, and has shown 

 that the villi of the villous layer are rooted in these canals. 



A morphologically quite distinct structure is the capsular 

 membrane of the perch. This, according to Mark and 



