GENERAL BIOLOGY OF EGGS 



15 



rabbit egg (Figs. 9 and 10). Variation in egg size is considered to be 

 attributable largely to differences in the content of non-living yolk 

 materials, but differences in nuclear size suggest that the amount of 

 active cytoplasm also varies. The eggs of the placental mammals 

 measure 60 to 180 \x in diameter (vitellus alone), those of rodents 

 occupying the lower part of the range. The egg of the field vole 

 Microtus agrestis (Fig. 24) is the smallest mammalian egg so far 

 recorded (Austin, 1957b). Very occasionally, 'giant' eggs are found, 

 which are 30 to 40 per cent larger in diameter 

 than normal ; these have been described in the 

 rabbit, rat, mouse (Austin and Braden, 1954c; 

 Austin and Walton, i960) and cotton-rat 

 (Austin and Amoroso, 1959) (Fig. 11). The 

 egg of the Australian native cat Dasyurus is 

 of notably larger dimensions, namely 240 /x in 

 diameter, but much the largest mammalian 

 eggs are those of the oviparous monotremes, 

 the spiny anteater Tachyglossus and the duck- 

 billed platypus Ornithorhynchus, in which the 

 vitellus at ovulation measures 3*5 to 4 mm. in 

 diameter (Flynn and Hill, 1939). Sea-urchin 

 eggs (Arbacia) are much the same size as rodent 

 eggs, the vitellus having a mean diameter of 

 74 jit (Harvey, 1956). By comparison, fish eggs 

 vary between 400 \jl and 150 mm., and frog eggs between 700 fi 

 and 10 mm. (Bcatty, 1956a). On the other hand, the egg of the 

 bryozoan Crista is only about 18 it in diameter and the oval eggs 

 of the parasitic worms Ascaris and Clouorchis have diameters of 

 about 60 and 45 jit, and 28 and 14 /x, respectively. Further informa- 

 tion on egg size is given by Hartman (1929), Boyd and Hamilton 

 (1952), Beatty (1956a), Costello et a\. (1957), Austin (1961a). 



The eggs of placental mammals, with volumes between 100,000 

 and 3,000,000 /x 3 , and that of Dasyurus, with a volume of about 

 7,000,000 ft 3 , are very big compared with most tissue cells, of which 

 the volumes lie between 200 and 15,000 /jl 3 . A motor neurone in a 

 large mammal, however, would have a volume of the order of 

 10,000,000 /x 3 , mainly on account of its remarkably long axon. The 

 smallest mammalian cells are probably the red blood cells and 



Fig. 11 

 Normal and 'giant' eggs 

 of the cotton-rat. X 220. 



spermatozoa, 

 respectively. 



the volumes of which are about ioo /x 3 and 30 



