7i8 LIFE : OUTLINES OF GENERAL BIOLOGY 



amount of living matter. The egg of the frog is iV inch in diameter, 

 and that is counted as large; for in many mammals t^u inch is a 

 common size, and the lancelet's is ^^^^r inch. Yet a shark's egg-cell, 

 hugely bloated with yolk, may be as large as an orange, and the 

 largest cell in the world is the "yolk" of an ostrich's egg, on the top 

 of which there lies a nucleated drop of living matter, the essential 

 part of the extraordinarily inflated cell. Even a small egg-cell, say 

 TO of a pin's head, may be a thousand times larger than a sperm- 

 cell; indeed, a thousand spermatozoa may be rushing round in the 

 investing semi-fluid film. In some cases the sperm-cell is ToxrVuo 

 times the size of the egg-cell. Yet the more important fact is that 

 the ripe egg-cell and the ripe sperm-cell contain in their nuclei the 



Fig. io8. 



A Spermatozoon (S), showing the head with the nucleus and the locomotor 

 tail. In proportionate magnification, a typical ovum: C, cytoplasm; N, 

 nucleus; CHR, chromosome; GS, germinal spot or nucleolus. A sperm 

 may be 1/100,000 the size of an egg-cell the size of a small pin's head. 



same number of chromosomes. The egg-cell has more building 

 material, but not more inheritance. 



The colloidal cytoplasm of the egg is not always homogeneous, 

 but shows some visible localisation. There are areas differing in 

 concentration and in rate of metabolism, and these are sometimes 

 marked by difference of colour in the living egg-cell, as in some 

 Tunicates. But while these may be normally concerned with supply- 

 ing building material for certain organs of the prospective body, 

 they are not separable in a hard and fast way. They are not defined 

 like the patches on a harlequin's coat. There appear to be delicate 

 films vaguely separating different regions of the cytoplasm, but the 

 detailed reticular or fibrillar or other microscopic structure that 

 can be seen in fixed and stained egg-cells is believed to be due to 

 post-mortem changes. By using different methods it is possible to 

 see different detailed structure in the same kind of egg, which 

 indicates clearly that the nets and fibrils and so forth are artefacts. 



