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 



30.0 



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 0.06 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 0.25 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 0.005 mm. long, possesses only an 

 exceedingly small fraction of the volume of a mammal ovum. Therefore, 

 to approach more closely the full range of cell size in the vertebrates, the 



