THE EARLY DEVELOPMENT OF THE MARSUPIALIA. 117 



ceeding at once to form the blastocyst wall, they are under 

 the necessity of keeping together, and to this end cleavage 

 has become adapted. For the ancestral radial arrangement 

 of the blastomeres in the 4-celled stage, charactei'istic of the 

 Monotreme and Marsupial, there has been substituted a 

 cross-shaped grouping into two pairs, and, as the outcome of 

 this adaptive alteration in the cleavage planes, there results 

 from the subsequent divisions, not an open cell-ring, as in the 

 Marsupial, but a compact cell-group or morula. In this we 

 again encounter precisely the same differentiation of the 

 blastomeres into two categories, respectively formative 

 (embryonal) and non-formative (trophoblastic) in significance, 

 as is found in the 16-celled stage of the Marsupial, but, since 

 tiie two groups of cells are here massed together, and in the 

 absence of any firm enclosing sphere, cannot spread inde- 

 pendently so as to form directly the wall of the blastocyst, 

 there has arisen the necessity for yet other adaptive modifi- 

 cations. Attention has already been directed to the tardiness 

 of differentiation in the embryonal region of the Monotreme 

 and Marsupial blastocyst, and here in the minute Eutherian 

 morula we find what is, perhaps, to be looked upon as a 

 further adaptive exaggeration of this same feature in the 

 inertness which is at first displayed Ijy the formative cells, 

 and which is in marked contrast with the activity shown by 

 the non-formative ectodermal cells. ^ It is these latter, it 



' The inertness of the formative cell-mass is accounted for by Assheton 

 ('98, p. 251) as follows : " Now, as the ei)iblast plays the more prominent 

 part in the formation of the bulk of the embryo during the earliest 

 stages, it clearly would l^e useless for tlie embryonic part to exhiljit 

 much energy of growth until the old conditions [in particular sufficient 

 room for embryonal differentiation] were to a certain extent regained ; 

 hence the lethargy exhibited by the embryonic epiblast in mammals 

 during the first week of development. No feature of the early stages of 

 the mammalian embryo is more striking than this inertness of the 

 embryonic epiblast — or, as I should now prefer to call it, simply epiblast 

 — during the first few days." Assheton, it should be remembered, holds 

 that the inner cell-mass of Eutheria furnishes only the embryonal 

 ectoderm. 



