THE EAKLY DLIVELOPMENT OF THE MARSUriALlA. 91 



regard the outer layer of the Monotreme blastocyst as 

 ectodermal. Hubrecht's view is that the primitive entodermal 

 cells of Semou give origin, uot to yolk-entoderm, but to the 

 equivalent of the embryonal knot of Eutheria, whilst tlie 

 uuihiminar blastodermic membi-ane itself is a larval layer 

 — the troplioblast — that portion of it overlying the internally 

 situated cells representing the covering layer (E-auber's layer) 

 of the Eutlierian blastocyst, "For this view/' remarks 

 Asshetou ['09, p. 233), "I can see no reason derivable from 

 actual specimens described and figured by those four authors" 

 (Caldwell, tSemon, Wilson and Hill), with which criticism I 

 am in entire agreement, as also with the following statement, 

 which, so far as the Metatheria are concerned, is based on 

 my own results: "Neither in the Prototheria [n ] or the 

 Metatheria is there really any tangible evidence of a tropho- 

 blast occurring as a covering layer over the definitive epiblast 

 as in Eutheria" (p. 234). 



In connection with the peripheral growth of the unilaminar 

 blastoderm in the Monotreme, it is of interest to observe that 

 this takes place, not apparently in intimate contact with the 

 surface of the solid yolk, as is the case with tlie growing- 

 margin of the extra-embryonal ectoderm in the Sauropsidan 

 egg, but rather in contact with the inner surface of the 

 thickened zona, perhaps as the result of the accumulation in 

 the perivitelliue space of fiuid which has diffused into the latter 

 from the uterus. In other words, the peripheral growth of 

 the extra-embryonal ectoderm to enclose the yolk-mass appears 

 to take place here in precisely the same way as the spreading 

 of the non-formative cells in Dasyurus to complete the lower 

 pole of the blastocyst. In my view the latter phenomenon 

 is none other than a recapitulation of the former ; on the 

 other hand, I regard the spreading of the formative cells in 

 Dasyurus towards the upper pole as a purely secondary 

 feature, conditioned by the loss of the yolk-mass and the 

 attainment of the holoblastic type of cleavage. 



If it be admitted that the outer extra-embryonal layer of 

 the Monotreme blastocyst is homogenous with the extra- 



