36 . J. p. HILL. 



or inner cell-mass of the Eutherian blastocyst. The non- 

 formative or extra-embryonal region directly gives origin to 

 the outer extra-embryonal layer of the bilaminar blastocyst 

 wall, i.e. to that layer which in the Sauropsida and Proto- 

 thoria is ordinarily termed the extra-embryonal ectoderm. I 

 regard it as such, and as the homologue of the so-called 

 trophoblast (or as I prefer to term it, the " trophoblastic 

 ectoderm " or " tropho-ectoderm ") of the Eutherian blasto- 

 cyst. 



A word or two here before concluding this section by way 

 of summary, as to the condition of the enclosing egg-envelopes. 

 During the sojourn of the eg^ in the uterus the albumen is 

 gradually resorbed, and by about the 16-cell stage it has all 

 but completely disappeared, thus permitting the zona to come 

 into direct apposition with the inner surface of the shell- 

 membrane. The shell-membrane itself increases very con- 

 siderably in tln'ckness during cleavage, and by the 16-celled 

 stage had practically reached its maximum, viz. "0075- 

 •008 mm., i.e. it is nearly five times thicker than that of the 

 ovum which has just entered the uterus. The thickened 

 shell-membrane by itself is firm and resistant, and it becomes 

 still more so by the application of the zona to its inner surface, 

 the two together forming a spherical supporting case round 

 the segmenting egg, to the inner surface of which the blasto- 

 meres become closely applied. 



The existence of such a firm supporting envelope round 

 the Marsupial egg is, in my view, a feature of very great 

 ontogenetic significance, and one which must be taken into 

 account in any comparison of the early developmental occur- 

 rences in the Metatheria and Eutheria. As the sequel will 

 show, the mode of formation of the blastocyst in these two 

 sub-classes is fundamentally different, and in my opinion 

 the explanation of this difference is to be found in the 

 retention by the Metatheria of a relatively thick resistant 

 shell-membrane, and its complete disappearance amongst the 

 Eutheria. 



