14 



THE MAMMALIAN EGG 



or attachment (mouse 5 days, rat 5-6 days, guinea-pig and man 6-7 

 days, rabbit and ferret 7-8 days, monkey 9-1 1 days, pig about 11 

 days, dog and cat 13-14 days, sheep 17-18 days, cow 30-35 days, 

 horse 8-9 weeks, animals with delayed implantation 8-9 months 

 or longer) (see Pincus, 1936a; Amoroso, Griffiths and Hamilton, 

 1942; Amoroso, 1952; Beatty, 1956a; Eckstein, 1959). 



100 200 300 400 500 600 700 800 900 1000 



Fig. 10 

 Sizes of mammalian eggs; pronuclear eggs except for Nos. 3 and 6. 1. The spiny anteater 

 Tachyglossus. 2 and 3. The Australian native cat Dasyurus. 4. The American opossum 

 Didelphis, at 24 hr after coitus; the disposition of the albumen layer and shell membrane at 

 72 hr is also indicated. 5 and 6. Rabbit eggs at 10 hr and 72 hr after ovulation, respectively. 

 7. Sheep. 8. Man. 9. Golden hamster. 10. Field vole. A = albumen layer. M = mucopro- 

 tein layer. S = shell. SM = shell membrane. Zp = zona pellucida. 



Size 



The sizes of mammalian eggs are by no means proportional to 

 the sizes of the adult mammals: the horse's egg is rather less than 

 twice the diameter of the mouse egg and about the same size as the 



