. Comparative anatomy. Anatomy, Comparative. 42 COMPARATIVE ANATOMY external to which may be a cellular membrane (corona radiata), both contributed by the ovary. Relative Size Comparison of sizes of eggs with little yolk and eggs with much yolk becomes more impressive if stated in terms of volume rather than diameter. The following data illustrate this: Egg of Amphioxus Some frogs Domestic fowl ("yolk") Approximate diameter, mm. o. I 2 .o Relative volumes 8,000 2 7,000,000 The volume of an ostrich ovum would be hundreds of millions of times greater than that of a mouse egg whose
. Comparative anatomy. Anatomy, Comparative. 42 COMPARATIVE ANATOMY external to which may be a cellular membrane (corona radiata), both contributed by the ovary. Relative Size Comparison of sizes of eggs with little yolk and eggs with much yolk becomes more impressive if stated in terms of volume rather than diameter. The following data illustrate this: Egg of Amphioxus Some frogs Domestic fowl ("yolk") Approximate diameter, mm. o. I 2 .o Relative volumes 8,000 2 7,000,000 The volume of an ostrich ovum would be hundreds of millions of times greater than that of a mouse egg whose diameter is about mm. The comparisons become more significant if every ovum, big or little, is to be regarded as structurally a single cell. There is, perhaps, a little room for question as to whether one of these immense yolk-laden ova, in its entirety, can be properly regarded as a cell. Such an ovum exhibits the extreme limit of that process of polarization (already well advanced in amphibians) which results from in- crease in the amount of egg yolk. Therefore, viewed with reference to its Fig. 33.^Human ovum surrounded supposed evolutionary history, it is by follicular cells. Actual diameter of • i ^ ^ i m • ovum about mm. c, cytoplasm equivalent to an amphibian ovum containing some yolk; CR, corona which is unquestionably a Cell. But, radiata: F, follicular cells: N, nucleus; ^1 . 1 • /• n 1 ZP, zona peiiucida. (After Nagel.) thinking of a Cell as a dynamic proto- plasmic unit, it is only the germ disc which is organized ''living" protoplasm. In a physiological sense, it is only the germ disc which is a cell. The whole ovum, however, can be regarded as structurally analogous to a fat cell, a cell distended by a relatively enormous globule of fatty material. But the spermatozoon is also essentially a cell. A rabbit spermatozoon, for example, with a "head" about mm. long, possesses only an exceedingly small fraction of the volume of a
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