THE EARLY DEVELOPMEXT OF THE MAESUPIALIA. 89 



SO as to enclose the entire yolk-mass in a complete ectodermal 

 envelope, whilst internally to that a complete lining of yolk- 

 entoderm has become established. As the result of these 

 changes, and of the imbibition of Huid from the uterus, the 

 solid yolk-laden egg has become converted into a relatively 

 thia-walled vesicle or blastocyst, possessed of a bi laminar 

 wall surrounding the partly fluid vitelline contents of the egg. 

 Throughout the greater part of its extent the structure of the 

 vesicle wall is very simple. It consists externally of an 

 extremely attenuated ectodermal cell - membrane closely 

 adherent to the deep surface of the vitelline membrane 

 (zona), and within that of a layer of yolk-entoderm, composed 

 of large swollen cells, containing each a vesicular nucleus, 

 and a number of yolk-spheres of varying size. Over a small 

 area, overlying the white yolk-bed, however, the ectodermal 

 layer of the wall presents a different character to thut 

 described above. Its constituent cells are here not flattened 

 and attenuated, but irregularly cuboidal in form and much 

 more closely packed together; moreover they stand in pro- 

 liferative continuity with a subjacent mass of cells, also in 

 process of division. The irregular superficial layer and this 

 latter mass together form a thickened lenticular cake, "o mm. 

 in greatest diameter, projecting towards the white yolk-bed 

 but separated from it by the yolk-entoderm, which retains 

 its character as a continuous cell-membrane. This differen- 

 tiated, thickened area of the wall, situated as it is at the upper 

 pole of the egg, as marked by the white yolk-bed, must be 

 held to represent a part of the future embryonal i-egion. 

 Wilson and Hill incline to regard it as in some degree the 

 equivalent of the "primitive plate" of Eeptiles and as the 

 initial stage in the formation of the primitive knot of later 

 eggs. This question, however, does not closely concern us 

 here : the point I wish to emphasise is the relative inactivity 

 of the cells composing the embryonal region of the blastoderm 

 in the Monotreme as compared with the marked activity dis- 

 played by those constituting the peripheral (extra-embryonal) 

 region of the same. It is these latter cells which by their 



