MUSEUM OF COMPARATIVE ZOOLOGY. 57 



that Midler has seen the equivalent of what I have called the micro- 

 pylar plug of granulosa cells, and it is therefore probable that he was 

 the first person to observe that peculiar structure in any fish-like animal. 

 If he were less positive in his assertion that the infolding contained an 

 opening, I should question if the cells took the form of a hollow funnel ; 

 even as it is, I doubt if the membrana propria is infolded. 1 



The first account of the membranes in Petromyzon Planeri was by 

 Max Schultze ('56, pp. 1-5). When taken from the body, the eggs had 

 besides the yolk membrane a firm "Eischalenhaut," or "chorion," which 

 was surrounded with a scarcely discernible thin layer of gelatinous sub- 

 stance, which was quickly swollen, when it came in contact with water, 

 to a thickness of not more than a quarter of a line. It was delicate and 

 fugitive, and was easily removable from the firm underlying membrane. 

 In the course of eight days it mostly disappeared, being dissolved in the 

 water ; it was not an " albuminous layer," but was rather to be compared 

 to the gelatinous mass uniting frogs' eggs ; its chemical composition was 

 not known. 



The firm " Eischalenhaut," which closely enveloped the egg, was a 

 clear membrane about 0.0015 m (probably should be 0.015 //; , or about 

 0.03 mm.) thick, which had a tendency after being torn to roll in at 

 the edges. It appeared very finely punctate when viewed from either 

 the inner or the outer surface. Schultze was inclined to regard the 

 punctations as due to very fine canals traversing the membrane, but 

 on account of the delicacy of the object he could not reach a perfectly 

 satisfactory conclusion on this point. For this finely punctate mem- 

 brane and that found in bony fishes, the author would use the name 

 chorion rather than vitelline membrane, for a true vitelline membrane 

 (or egg-cell membrane) exists inside the punctate structure. 



Owsjannikow ('70% p. 184) says that the gelatinous layer of the outer 

 egg membrane is very little developed, so that the fertilized eggs are 

 only feebly attached to the objects on which they fall, the least current 

 carrying them away. 



Calberla's ('78, pp. 438-441) account is in some particulars more 

 extended than that of Schultze. The eggs, he says, instead of being 

 round, are slightly ellipsoidal. The membrane (zona) consists of two 

 layers, which are not, however, sharply separated from each other. The 

 outer is highly refractive, rough externally owing to all sorts of eleva- 

 tions and tooth-like structures (Zacken) ; the inner is much thinner 



1 For a review of more recent work on Myxine, see pp. 91-93, 107-110. 



