102 THE MAMMALIAN EGG 



slower and may take as long as 17 hr (Pincus, 1930); mucin deposi- 

 tion would be similarly delayed. These considerations give force to 

 Chang's (195 id) contention that the demonstrably short fertilizable 

 life of the rabbit egg should not be ascribed to its acquisition of a 

 mucin coat. 



When the rabbit blastocyst expands in the uterus, the mucin coat, 

 as Boving (1954) points out, is reduced to a thickness of only a few 

 microns, while the zona pellucida must become vanishingly thin. 

 Boving found, nevertheless, that the rabbit blastocyst is surrounded 

 by two distinct membranes and he suggests that the outer mem- 

 brane, which he calls the 'gloiolemma', is secreted by the uterus and, 

 by virtue of its adhesive property, is intimately involved in the 

 implantation reaction. 



Outer Coats of Marsupial and Monotreme Eggs 



The eggs of the opossum Didelphis (Hartman, 1916, 1919; Hill, 

 191 8), the native cat Dasyurus (Hill, 1910) and the wallaby Setonix 

 (Sharman, 1955a) acquire a coating of jelly-like material in their 

 passage through the Fallopian tube (Fig. 10); this is referred to as 

 albumen although its chemical nature does not seem to have been 

 investigated. In the opossum, more albumen is added in the uterus, 

 the final thickness of the coat amounting to rather more than the 

 original diameter of the egg. Both opossum and native-cat eggs 

 receive in addition a shell membrane, which becomes thicker with 

 time. The opossum egg is also described as having a shell, but this 

 is non-calcareous and leathery in texture. 



The eggs of the monotremes, the duck-billed platypus Ornitho- 

 rhynchus and the spiny anteater Tachyglossus (= Echidna), resemble 

 bird and reptile eggs rather than those of marsupials and placental 

 mammals (Caldwell, 1887; Gatenby and Hill, 1924; Flynn, 1930; 

 Hill, 1933 ; Flynn and Hill, 1939) ; they become covered with a broad 

 layer of albumen, a shell membrane and a leathery shell (Fig. 10). 



