186 WILLIAM WALLACE. 



We see, therefore, that immature ova, as compared with 

 ripe egg's of Zoarces, possess an extra membrane, the 

 zonoicl layer; that this layer arises as a differentiation of 

 the superficial protoplasm ; that it becomes distinct from the 

 vitellus, and then consists of radial fibrils supported above 

 and below by thin surfaces — quite separate from either the 

 vitellus or the egg membrane proper ; that is to say, it is 

 now a distinct layer. It follows from this, and from the 

 obvious fact that the radial fibrils are far more numerous 

 than the cells of the follicular epithelium, that the zonoid 

 layer cannot be " due to the partial withdi'awal, through 

 contraction, of the -protoplasm occupying the pore canals of 

 the egg membrane," as Eigenmaun supposed. It is to be 

 carefully distinguished from a true egg membrane, because, 

 so far as can be made out from microscopic examination, it is 

 a "living" membrane, the fibrils of cj'toplasm of which it is 

 composed not having undergone any chemical metamorphosis, 

 nor apparently do the spaces between the fibrils contain any 

 substance. The zonoid layer seems to disappear in the ripe 

 egg. My own impression is that during the great and rapid 

 increase in the volume of the egg at maturation the zonoid 

 layer is stretched out under the egg membrane, and is so 

 attenuated as to be invisible in ordinary preparations. 



Identical appearances were seen in surface sections of 

 immature ova of Sygnathus acus, Salmo Levenensis,^ 

 and Pleuronectes platessa. The eggs of these four 

 species — and no doubt of many other Teleosteans — possess a 

 single definitive egg membrane or zona radiata, while 

 below this, in immature ova, is the zonoid layer. I think 

 these eggs should all be placed in Eigenmann's first category, 

 i. e. " (1) a. Zona a single layer of uniform structure." 



The investments of the ovarian egg of the John Dory — 

 Zeus faber (fig. 34) — are thicker and more complex. 

 There are three distinct membranes, or, if we include the 

 zonoid layer, which is, as usual, present in the immature 



1 I have totliaiik Dr. II. M. Kyle for llie ovary of tlie Loch Leveii trouf. 



