GENERAL BIOLOGY OF EGGS 



13 



embryo becomes attached to or embedded in the uterine mucosa. 

 As a rule, fertilization begins and ends in the ampulla of the 

 Fallopian tube, but there are some exceptions : in the tenrecs (primi- 

 tive insectivores of Madagascar), sperm penetration occurs while the 

 eggs are still in the ovary and they pass to the tube during pro- 

 nuclear development (Bluntschli, 1938; Strauss, 1938, 1950). 

 Penetration within the follicle has also been said to take place in the 

 noctule bat (Van der Stricht, 1909), and the shrew (Stratz, 1898, 

 cited by Strauss, 1954; Pearson, 1944), and even, according to some 

 early investigators, in the rabbit and dog (Barry, 1839; Bischoff, 

 1842a). The eggs of most mammals can wait for little more than 

 12 hr if fertilization and development are to occur in a normal 

 manner (see Hartman, 1924; Blandau and Young, 1939; Chang, 

 1952b; Blandau, 1954; Braden and Austin, I954d; Laing, 1957). In 

 the native cat Dasyimis, the opossum Didclphis, the wallaby Setonix 

 and the spiny anteater Echidna, the eggs pass into the uterus whilst 

 still in the pronuclear stage (Hill, 1910; Hartman, 1928; Flynn and 

 Hill, 1939; Sharman, 1955a, b). Passage through the Fallopian tube 

 may take only 24 hr, as in the monotremes and marsupials, or 2 to 

 3 days, as in rodents, but in most other mammals the interval is 



Fig. 9 



Sizes of animal eggs (vitellus alone). 

 The horizontal lines show the upper and 

 lower limits for the eggs of marsupials 

 and placental mammals, (a) Outline of 

 the monotreme egg. (b) Some of the 

 largest invertebrate eggs, such as those of 

 the squid Loligo, the gastropod Bitsycon, 

 the starfish Henricia, and the crabLibinia. 

 (c) The smallest frog eggs, (d) The smallest 

 fish eggs, (e) The Australian native cat 

 Dasyurus and also the sea-squirt Amarou- 

 cium. (/), (g) and (//) The sizes of the 

 majority of mammalian eggs and also of 

 those of many echinoderms, tunicates, 

 molluscs, polychaets, nemcrtines, platy- 

 helminths and coclenterates. Sheep, cow, 

 dog and horse eggs are represented by 'f ', 

 human, rabbit and cat eggs by 'g' and 

 most rodent eggs by 'h'. (/) The smallest 

 mammalian egg, that of the field vole 

 Microtus agrestis; also the egg of the clam 

 Spisitla. (j) The smallest animal eggs, 

 including that of the bryozoan Crista. 



\00ju 



I mm 



between 4 and 8 days. Species differences are seen in the rate of 

 cleavage of the early embryo, p. 83, and in the time of implantation 



