THE EARLY DEVELOPxAIENT OF THE MARSUPIALIA. 79 



ectoderm on the left side of fig. 82, there is visible a small 

 spherical cell similar to the degenerate cells met with in 

 blastocysts of Dasyurus. 



My last stage of M. ruficollis comprises an excellently 

 preserved blastocyst, measuring "8 mm. in diameter, in which 

 the embryonal ectoderm and the entoderm are definitely 

 established. It thus coi-responds to the 8, '01 stao-e of 

 Dasyurus (blastocysts 5-5'5 mm. diameter). The embryonal 

 area is circular and measures "468 mm. in diameter. Its 

 constituent cells are cubical and from '008 to 'OlS mm. in 

 thickness, Avhilst the trophoblastic ectoderm is formed of 

 flattened cells, -006 mm. in thickness. The entoderm is 

 present as a continuous layer of attenuated cells below the 

 embryonal ectoderm, and it probably also forms a continuous 

 layer below the trophoblastic ectoderm. Entodermal cells are 

 certainly present over the lower polar region of the vesicle, 

 but it is difficult to be certain from the sections whethei- or not 

 they form a perfectly continuous layer. The shell membrane 

 has a thickness of "0026 mm. 



I have a corresponding blastocyst of Petrogale peni- 

 cillata 'QLo mm. in diameter, with an oval, embr^^onal area 

 •52o X "45 mm. in diameter, and a later blastocyst of M. 

 ruficollis 1'4Q mm. in diameter, with a circular embryonal 

 area '57 mm. in diameter. 



Chapter VI. — General Summary and Conclusions. 



The observations recorded in the preceding pages and the 

 conclusions deducible therefrom may be summarised as 

 follows : 



(a) Ovum. — The uterine ovum of Dasyurus is characterised 

 (1) by its large size relatively to those of Eutheria ; (2) by 

 the presence externally to the zona of a layer of albumen and 

 a shell-membrane, both laid down in the Fallopian tube and 

 homologous with the corresponding- structures in the Mono- 

 treme ovum, the shell-membrane, like the shell of the latter, 

 increasing in thickness in the uterus; (3) by its marked 



