54 BULLETIN OF THE 



B. Historical and Critical Review of the Literature on the 

 Primary Egg Membranes 1 and the Micropyle in Fishes. 2 



It is possible that the eggs of fishes may present as many as four 

 essentially distinct kinds of enveloping membranes before separation 

 from the ovary. The innermost of these, if it exists, may be considered 

 a true vitelline membrane, the equivalent of the cell membrane in general. 

 I have made no observations concerning it, and shall have little to say 

 regarding the conflicting testimony as to its existence. The second, pro- 

 ceeding from the yolk outward, is radially striate, and I shall call it, as 

 in the preceding description, zona radiata. Although this is totally dif- 

 ferent in structure from the next membrane, there are several reasons 

 why it will be best to consider both at the same time. This third 

 membrane I shall call, as previously, the villous layer. The fourth and 

 outermost, when it exists, is formed exclusively from the granulosa cells, 

 and may be called by the name first given to it by Johannes Miiller, — 

 capsular membrane} 



a. Cyclostomata. 



The eggs of the myxinoids are enveloped in a " horny capsule," which 

 was first described by Thomson ('59, pp. 50, 51) for Myxine glutinosa. 

 He evidently considered it the equivalent of the egg cases of selachians. 

 Since the latter are formed in the oviduct, they cannot be considered 



1 I use the expression primary egg membranes in the sense in which it has been 

 employed by Ludwig ('74 p. 197), i. e. for all membranes which are the product of 

 either the ovum itself or the follicular epithelium surrounding it. 



2 Owing to delays in publishing my studies I have been able to extend this 

 review, and to bring it down so as to include papers which have appeared since 

 my own account was written. 



3 I have the less hesitancy in adopting this name because Miiller ('54, p. 189) — 

 notwithstanding some misconceptions as to its real nature in the perch — gave the 

 following concise, and, in my opinion, still perfectly applicable definition: "Eine 

 von dem Eifollikel, Ovisac eines Wirbelthiers erzeugte Eihiille scheint von der 

 Eischale anderer Eier unterschieden werden zu mussen als capsulare Eihiille, oder 

 Eicapsel." When subsequent observers, — as for example His ('73), — ignoring the 

 true explanation of Midler's investigations given by Leuckart ('55, pp. 257-260), 

 transfer the name Eicapsel to the zona radiata, one is compelled to protest that 

 that was not the structure described by Miiller under the name of "Eicapsel," and 

 that no one has yet brought forward satisfactory evidence that the zona is "pro- 

 duced by the egg follicle," as Midler's definition demands. It therefore seems to 

 me that it is better, for the sake of avoiding confusion, to drop entirely the name 

 capsule — whether egg capsule or "cartilage capsule" (His) — as a designation 

 for the zona radiata. 



