42 THE OVUM AND 



by a very delicate pellicle, the albumen-membrane, b, also that 

 the transparent substance itself (albumen) is sufficiently fluid to 

 permit of such a degree of displacement of the yelk as to allow 

 of its coming into contact even with the albumen-membrane. 

 Although I have never yet succeeded in observing this pellicle, 

 and though in my researches the transparent membrane, on 

 the bursting of the yelk, always tore with smooth edges like a 

 solid substance, yet the observations of the respected discoverer 

 are too precise to admit of a doubt upon it. It is also sup- 

 ported by the analogy of most of the ova of other classes of 

 animals, in which chorion and vitelline membrane may gene- 

 rally be distinguished, notwithstanding that they sometimes lie 

 close upon each other. The albumen-membrane has probably 

 the signification of a cell-membrane, in which case the albumen 

 will be the cell- contents, and the yelk a young cell. Accord- 

 ing to Wharton Jones, the transparent areola (zona pellucida) 

 of the ovum, or the albuminous layer in the fecundated 

 ovum of mammalia, becomes considerably expanded in the 

 tubes, a fact which would be readily explained by the inherent 

 energy of the albumen-membrane when regarded as a cell. 

 In such case, however, the mode of formation of the albumen 

 would be very different from the corresponding process in the 

 bird's egg, where, according to Purkinje, it is secreted by the 

 oviduct, and a membrane (chorion) is formed around it sub- 

 sequently, which cannot therefore have the signification of a 

 cell-membrane, and is moreover not simple in structure, but 

 composed of fibres. Meanwhile an investigation might be 

 made, as to whether the albumen in the egg may not also be 

 first surrounded and formed by an equally thin pellicle, 

 around which a secondary external membrane may subsequently 

 be produced. According to Purkinje, however, this is not 

 the case, and I could not discover any such pellicle upon the 

 inner surface of the shell-membrane of the excluded egg. I 

 have not made any inquiry as to whether the chorion of fishes 

 is a cell-membrane or not. It is covered internally with a 

 very beautiful epithelium, which is made up of more or less 

 flat hexagonal cells, each of which has its nucleus. 



Within the transparent areola, or, according to Krause, the 

 albuminous layer, lies the vesicle of Baer, or the yelk ; which, 

 from Krause's statement, is enclosed by a peculiar structureless 



