THE EARLY DEVELOPMENT OF THE MARSUPL\LL\. 23 



must pass with considerable rapidity down the tubes to the 

 uteri where cleavage begins, for I have only once found a 

 tubal ovum, and that one had evidently been retarded for 

 some reason, and was polyspermic. 



S. The Secondary Egg-membranes: Albumen and 

 S h e 1 1 •- m 6 m b r a 11 e . 



During the passage of the ovum down the tube it is 

 fertilised, and becomes enclosed externally to the zona by 

 two secondary layers formed as secretions by the cells of the 

 oviducal lining. First of all, the ovum becomes surrounded by 

 a transparent to semi-transparent laminated layer of albumen, 

 •015 to "022 mm. in thickness, composed of numerous very 

 delicate concentric lamellas, and having, normally, numbers 

 of sperms imbedded in it (figs. 8-11, alh., sp.). Then outside 

 the albumen layer there is laid down a definite, but at first 

 very thin, double-contoured membrane (figs. 8 and 10, s.m.), 

 wliicli, following Caldwell, I have no hesitation in lioino- 

 logising with the shell-membrane of the Monotreme egg. 

 Caldwell in 1887 described and figured a definite membrane 

 enclosing the uterine ovum of Phascolarctus, externally to, 

 and quite distinct from the albumen, which he interpreted 

 as the representative of the shell-membrane of the Mono- 

 tremata, but owing apparently to the fact that Selenka 

 altogether failed to recognise its true nature in Didelphys, 

 since he regarded it as a derivative of the follicular epithe- 

 lium, and termed it the " granulosa-membran,'' this highly 

 significant discovery of Caldwell has been largely ignored. 

 Such a membrane is constantly present and easily recognisable 

 in all the Marsupials (Dasyurus, Perameles, Trichosurus, 

 Macropus, Petrogale, Phascologale, Acrobates, Phascolarctus, 

 Bettongia), of which I have had the opportunity of studying 

 early developmental stages. It is laid down in the Fallopian » 

 tube, is perfectly distinct from the albumen, and increases in 

 thickness in the uterus, and if it has not the significance 

 which Caldwell has suggested, then I must leave it to those 



