MUSEUM OF COMPAKATIVE ZOOLOGY. 147 



first appearance of the egg membrane. The absence of striae in these 

 younger eggs may be accounted for by assuming, as Reichert has sug- 

 gested, that the zona radiata is a later growth, and that the imperforate 

 membrane of younger eggs is a different structure, or that during the 

 earlier stages the material composing the membrane is less dense, al- 

 lowing the food material to have ready access to the yolk. Granting 

 for the moment that the zona grows by apposition of layers from within, 

 the latter view is the more probable, because in the perch the inner 

 portion of the zona is not perforate even after the outer is distinctly so, 

 and in most cases the pore-canals are much more distinct and wider in 

 the outer than in the inner portion of the zona. The meaning of the 

 pore-canals, in the intra-ovarian egg at least, needs little discussion. In 

 most of the sections prepared, where the granulosa cells are slightly 

 raised from the zona radiata, processes of the granulosa cells can be seen 

 to enter the pore-canals. 



Various membranes have been described for different fishes as over- 

 lying the zona radiata. The peculiar capsular layer of the perch has 

 been seen by all authors who have examined the eggs of this fish. It 

 was first described by Von Baer ('35). 



Rusconi ('36) describes a thin membrane overlying the ovarian eggs 

 of Cyprinus. Aubert ('53) saw an outer membrane on eggs of Esox 

 which had lain in water some time. Kolliker ('58) succeeded in isolat- 

 ing this membrane in the case of Esox. 



Reichert ('56) discovered that whenever processes are present, as 

 in many cyprinoids, they are set in a thin outer membrane. Kol- 

 liker ('58) confirmed this statement, and added that this outer mem- 

 brane is developed before the zona radiata. Reichert also found that 

 the membrane of the smallest membrane-bearing ovarian eggs is not 

 striate, and concluded that the zona radiata must be a secondary for- 

 mation. 



Vogt ('42) was the first to describe a membrane within the zona 

 radiata. He found that in the eggs of Coregonus palea and Salmo 

 umbla this membrane cannot be readily seen until after the eggs have 

 been in water for some time, and that it passes (p. 29) gradually into 

 the germ. Ransom ('56) found a similar structure in eggs of Gaste- 

 rosteus pungitius, in which this inner membrane takes part in cleavage. 

 Eimer ('72 a ) claims to have isolated this vitelline membrane, which he 

 saw in trout, pike, white-fish, and perch. Oellacher C72) also succeeded 

 in separating it in the brook trout. I believe that the structures de- 

 scribed by Vogt, Ransom, Eimer, and Oellacher are, as others have 



