THE DEVELOPMENT OF THE MOLE. 203 



zona by a slightly dilated mouth, while on the outer side of 

 the zona they communicate with the exterior by a considerably 

 wider opening (fig. 7). Into the external openings of these 

 canals I have been able to trace prolongations of those cells 

 of the discus which are immediately in contact therewith 

 (fig. 7), and there appears to me no room for doubt that the 

 contents of these follicular cells are thus rendered available 

 for the nutriment and growth of the ovum. 



Owing to the extreme minuteness of the canals it is quite 

 possible that they are only rendered visible by the protoplasm 

 of the follicular cells, which is less transparent than the zona 

 itself, passing through them, and the fact that careful observers 

 have not succeeded in detecting these pores would be accounted 

 for by the cessation of the nutrient process at the time of 

 observation. I may add I have observed the radial canals 

 through the zona in optical sections of various whole ova, as 

 well as in many actual sections of ova situated within the 

 Graafian follicle. 



I have before mentioned that the close investment of the 

 ovum with follicular epithelium cells is in accordance with the 

 degree of ripeness of the ovum itself. When the latter is fully 

 mature only a very small number of, and in some instances no, 

 epithelium cells are carried out with it upon the rupture of 

 the follicle. Thus the attachment of the epithelium to the 

 zona ceases when the ovum becomes mature, and no further 

 nutriment is required, and this is of itself some further proof 

 of the nutrient function of the follicular epithelium cells. 



I myself never detected any follicular cells within the zona, 

 such as has been described by Lindgren (No. 15), von Sehlen 

 (No. 21), and Virchow (No. 22) ; nor have I seen any trace of 

 a micropyle in the zona, such as M. Barry (No. 3) and others 

 held to exist. 



The Vitelline Membrane. 



Within the zona radiata and enclosing the ovum itself in all 

 those ripe ovarian ova examined by me, is a second very thin 



