THE BIOLOGY OF THE CELL SURFACE 



of the ^JT<2ru-spermatozoon, those of these crustaceans 

 present more slender projections resembling the finer pseu- 

 dopodia of an amoeba. These spermatozoa are highly 

 interesting because they are first inverted and explode at 

 fertilization.^ 



Spermatozoa despite these great variations in structure 

 have certain features in common: they possess very little 

 cytoplasm although the volume of this may be greater than 

 the nuclear volume. Compared to the egg, any species of 

 spermatozoon is extremely minute. One fact already 

 mentioned above is to be emphasized again: sperm-cells 

 capable of fertilizing eggs are always fully matured — that 

 is, they are always spermatozoa and never spermatocytes 

 or spermatids. 



Whereas most, probably all, species of animal sperma- 

 tozoa are motile, with few exceptions eggs are incapable of 

 locomotion. Whilst, moreover, animal eggs vary in size 

 from a few microns in diameter to several centimeters (e.g., 

 eggs of cursorial birds), the smallest species of eggs are 

 larger than the largest species of spermatozoa. This 

 greater bulk of the egg is due to its cytoplasmic mass with 

 inclusions, oil, yolk, mitochondria, etc. If these are sus- 

 pended in the cytoplasm they may amount to about two- 

 thirds of the egg-volume; if they are pendent to the cyto- 

 plasm, as in birds' eggs, for example, they make up all but an 

 extremely small fraction of the egg. No eggs, including 

 the so-called transparent ones, are entirely yolk-free. In 

 comparison with spermatozoa, animal eggs as a class at the 

 time of fertilization contain more cytoplasm and are richer 

 in reserve or food materials, yolk and oil. 



As was pointed out, eggs in contrast to spermatozoa can 

 be fertilized before, during or after their maturation, 

 depending upon species. We distinguish four classes of 



^ See especially Kolzoff, igio. 



