ADDRESS. lxxxi 



Usually more or less spherical in form, the animal ovum presents the 

 essential characters of a " complete cell," in the signification given by Schwann 

 to that term. The germinal substance is enclosed by an external vesicular 

 membrane or cell-wall. Within this coveriug the cell-substance (generally 

 named yolk or vitellus, from the analogy of the fowl's egg) consists, to a greater 

 or less extent, of a mass of protoplasm ; and imbedded in this mass, in a deter- 

 minate situation, there is found a smaller internal vesicular body, the germi- 

 nal vesicle or nucleus, and within that the somewhat variable macula or 

 nucleolus. 



Now the first thing which strikes us as remarkable connected with the 

 ovum is the very great variation in its size as compared with the entire animal 

 to which it belongs, while in all of them the same simple or elementary struc- 

 ture is maintained. The ovum of mammals, for example (discovered by Von 

 Baer in 1827) is a comparatively small body, of the average diameter of 

 about y^g- of an inch, and consequently scarcely weighing more than a 

 minute fraction of a grain, perhaps not more than the 1 2 1 ^ part. And 

 further, in two animals differing so widely in size as the elephant and the 

 mouse, the weights of which may stand towards each other in the proportion 

 of 150,000 to 1, there is scarcely any difference in the size of the mature 

 ovum. 



On the other hand, if we compare this small ovum of the mammal with 

 the yolk of the egg in the common fowl, the part to which it most nearly 

 corresponds, it may be estimated that the latter body would contain above 

 three millions of the smaller ova of a mammal. 



The attribute of size, however, in natural objects ceases to excite feelings 

 of wonder or surprise as our knowledge of them increases, whether that be 

 by familiar observation or by more scientific research. We need not, at all 

 events, on account of the apparent minuteness of the ovum of the mammifer 

 or of any other animal, have any doubts as to the presence of a sufficient amount 

 of germinal substance for explaining in the most materialistic fashion the 

 transmission of the organic and other properties and resemblances between the 

 parent and offspring. For we are led to believe, by those who have recently 

 given their attention to the size of molecules composing both living and dead 

 matter, that in such a body as this minute ovum of the mammal there may 

 be as many as five thousand billions of molecules ; and even if we restrict 

 ourselves to the smaller germinal vesicle, and, indeed, to the smallest germinal 

 particle which might be made visible by the highest microscopic enlargement, 

 there are still sufficient molecules for all the requirements of the most exact- 

 ing material biologist *. 



* According to a calculation made bj Mr. Sorby, the number of molecules in the ger- 

 minal vesicle of the mammalian ovum is such that if one molecule were to be lost in every 

 second of time, the whole would not be exhausted in seventeen years. See Address to the 

 Microscopic Society, in Journ. of Microscop. Science, vol. xv. p. 225, and ' Nature,' vol. 

 xiii. p. 332. See also Darwin on "Pangenesis," in his work on 'Variations,' &c. (18C8), 



