S6 ^ J. p. HILL. 



falls into line with that of Eutheria, and with what we know 

 of the early development of the Prototheiia, but furnishes 

 ns with the key to the correct interpretation of that extra- 

 ordinarily specialised developmental stage, the Eutherian 

 blastocyst. In particular I liope to show that the description 

 which I have been able to give of the mode of formation of 

 the Marsupial blastocyst, bridges in the most satisfactory 

 fasliion the great gap which has till now existed in our 

 knowledge of the Avay in which the transition from the 

 Monotrematous to the Eutherian type of development has 

 been effected. 



1. The Early Development of the Monotremata. 



Our knowledge of the early development of the oviparous 

 m;unin:ils is admittedly still far from complete. Nevertheless 

 it is not so absolutely fragmentary that it can be passed over 

 in any general discussion of early mammalian ontogeny, and 

 I certainly cannot agree with the opinion of Assheton ('08, 

 p. 227) that fi'om it "we gain very little help towards the 

 elucidation of Eutherian development." On the contrary, I 

 think that the combined observations of Semon ('94), and 

 Wilson and Hill ('07) shed most valuable light on the early 

 ontogenetic phenomena in both the Metatheria and Eutheria. 

 I propose therefore to give here a very brief resume of the 

 chief results of these observers,^ and at the same time to 

 indicate how the knowledge of early Monotreme ontogeny 

 we possess, limited though it be, does help us to a better 

 understanding of the phenomena to which I have just 

 referred. 



The ovum, as is well known from the observations of 



Caldwell ('87), is Reptilian in its character in all but size. 



It is yolk-laden and telolecithal, the yolk consisting of 



discrete yolk-spheres, and it is enclosed outside the zona 



(vitelline membrane) by a layer of albumen and a definite shell. 



' In so doin^' I li;ive largely utilised the phraseology of Wilson and 

 Hill's paper ('07). 



