REPRODUCTIVE ELEMENTS IN MYXINE ftLUTINOSA. 61 



at the poles of the follicle than in the equatorial region, and 

 the membrane in question varies in thickness with the epithe- 

 lium. I shall recur to the question of the nature and origin 

 of the membrane further on. In this paper I shall call the 

 membrane the vitelline membrane, using that term to include 

 the whole of the primary egg membranes produced within the 

 follicle. The terra chorion will not be used, as its application 

 in the case of mammals to a membrane which is partly derived 

 from the blastoderm renders it unsuitable in connection with 

 primary egg membranes. The term vitelline membrane, as 

 used in this paper, implies no assertion as to derivation from 

 follicular epithelium on the one hand, or ovum on the other. 

 In Myxine, as will be conclusively shown in the course of what 

 follows, the vitelline membrane forms the sole protective covering 

 of the deposited ovum. ^ 



It is now necessary to trace the destiny and elucidate the 

 significance of the process from the follicular epithelium above 

 described. In the sections succeeding the one shown in fig. 2 

 this cylindrical process is seen to penetrate the vitelline mem- 

 brane, occupying a tubular cavity in the latter, and passing 

 through it to form a hemispherical projection on its inner sur- 

 face. This tubular aperture in the vitelline membrane, with 

 its contained epithelial cells, is shown in fig. 3, as seen under a 

 high power. The section lies almost in the plane of the canal, 

 and so exposes nearly the whole of its cellular contents, in- 

 cluding the hemispherical projection surrounded by the proto- 

 plasm of the germinal disc. This cellular projection is covered 

 by a thin membrane continuous with the vitelline membrane, 

 and is not in immediate contact with the germinal disc. In 

 fig. 3 the outer end of the canal, that towards the follicular 

 epithelium, is closed, owing to the direction of the plane of the 

 section, but in previous sections, as stated above, the cellular 

 cylinder filling up the canal is seen to be continuous with the 

 process projecting from the surface of the follicular epithelium. 

 The structures now described, as I have convinced myself by 

 a series of sections from more than one egg, exist only at one 

 pole of the ovarian ovum. There is thus at one pole of the 



%f 



