THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 115 



lower mammals. In the cross-shaped arrangement of the 

 blastomeres in the 4-ceIled stage, in tlie occurrence of a 

 definite morula-stage and of the entypic condition, we have 

 features in which the early ontogeny of the Eutheria differs 

 fundamentally from that of the Metatheria. They are inti- 

 mately correlated the one with the other, and are met ^\•ith in 

 all Eutheria, so far as known, but do not occur either in the 

 Prototheria or the Metatheria, so that we must regard them 

 as secondary features which were acquired by the primitive 

 Eutheria under the influence of some common causal factor 

 or factors, subsequent to their divergence from the ancestral 

 stock common to them and to the Metatheria. Now the cross- 

 shaped 4-celled stage and the morula-stage are undoubtedly 

 to bo looked upon simply as cleavage adaptations of prospective 

 significance in regard to the entypic condition, so that the 

 problem reduces itself to this — how came these adaptations 

 to be induced in the first instance ? In view of the facts that 

 in the Metatheria, in the presence of the shell-membrane, the 

 formation of the blastocyst is the direct outcome of the cleavage 

 process, and is effected along the old ancestral lines without 

 any enclosure of the formative cells by the non-formative, 

 whilst in the Eutheria, in the absence of the shell-mem- 

 brane, blastocyst formation results only indirectly fi'om the 

 cleavage-process, is effected in a way quite different from 

 that characteristic of the Metatheria, and involves the 

 complete enclosure of the formative by the non-formative 

 cells, I venture to suggest that the cleavage adaptations 

 which result in the entypic condition were acquired in the first 

 instance as the direct outcome of the total loss by the already 

 greatly reduced Eutherian ovum of the shell-membrane.^ 

 This vieAv necessarily implies that the presence of a thick 

 zona such as occurs round the ovum in certain Eutheria is 

 secondary, and what we know of tin's membrane in existing 

 Eutheria is at all events not adverse to that conclusion. 

 ' This suggestion I first put forward in a course of lectures on the 

 early ontogeny and placentation of the Mammalia delivered at the 

 University of Sydney in 1904. 



