STRUCTURE OF THE OVARY GRAAFIAN FOLLICLES. 175 



the time of Purkyne's, v. Baer's, and R. Wagner's discoveries, 

 the protoplasm of the (primordial) ovum has been called the 

 vitellus (Eidotter), or in accordance with the judicious suggestion 

 of His, the chief or principal yolk (Hauptdotter). 



In reference to its behaviour in the process of development, 

 since it alone undergoes change, this yolk receives also the 

 name of formative yolk (Bildungsdotter). To indicate the 

 nucleus and nucleolus, the terms vesicula germinativa (Pur- 

 kyne's vesicle, keimblaschen), and macula germinativa 

 (Wagner's spot, Keimfleck), are commonly used. 



Already, whilst it is still retained within the Graafian 

 follicle, the primordial ovum becomes surrounded by a series of 

 secondary formations, that are collectively to be regarded as 

 products of the follicular epithelium. To these belong the 

 vitelline membrane (Dotterhaut, Eihaut), or as it is usually 

 called in the ova of Mammals, the zona pellucida, and the 

 food yolk (Nahrungsdotter), or according to His's nomenclature, 

 the secondary yolk (Nebendotter). When these last-named 

 parts are completely formed, we have the "ripe ovarian ovum" 

 before us, which now leaves its previous resting-place, the 

 follicle, in order to descend towards the point where it becomes 

 fertilized. During the passage of the mature ovarian ovum 

 through the genital canal, in the greater number of animals, 

 and especially in those in which the embryo first undergoes 

 further development external to the maternal organism, it 

 obtains a larger or smaller number of protecting investments, 

 that have been satisfactorily named by Reichert, oviducal 

 sheaths or membranes (Eileiterhiillen). 



Thus it appears that in order to give a correct explanation 

 of the various parts of the ovum, as well as to enable an accu- 

 rate comparison to be made between the ova of the various 

 classes of animals, we must distinguish between the primordial 

 ovum, the ripe ovarian ovum, and the ovum after it has 

 acquired its oviducal investments. 



It may be here just noted that those structures that are ordinarily 

 termed ova in the ovaries of Birds, Amphibia, Reptiles, Fishes, etc., 

 are not really the ova, but Graafian follicles. 



The investing membrane of the ovum, the zona pellucida 



