92 J. r. HILL. 



embryonal ectoderm of the Eeptile, then it seems to me there 

 is no escape from the conclusion that these layers are also 

 homogenous with the non-formative region of the unilaminar 

 Marsupial blastocyst. I need only point out here that the 

 chief destiny of each of the mentioned layers, and I might 

 also add that of the outer enveloping layer of the Eutherian 

 blastocyst (the so-called trophoblast), is one and the same, 

 viz. to form the outer layer of the chorion (false amnion, 

 serous membrane) and omphalopleure (unsplit yolk-sac wall, 

 Hill ['97]),' and that to deny their homogeuy to each other 

 implies the nou-homogeny of these membranes and the amnion 

 in the Amniotan series, and consequently renders the group 

 name Amniota void of all morphological meaning. 



The rapidity with which the enclosure of the yolk-mass 

 is effected, and the relative tardiness of differentiation in the 

 embryonal region are features which sharply distinguish the 

 early ontogeny of the Monotremes from that of the Sauropsida, 

 and which, in my view, are of the very greatest importance, 

 since they afford the key to a correct understanding of the 

 peculiar coouogenetic modifications observable in the early 

 ontogeny of the Metatlieria and Eutheria. To appreciate the 

 significance of these features it is necessary to take account 

 of the great difference which exists between the Sauropsidau 

 and Monotreme ovum in regard to size, as Avell as of the vevy 

 different conditions under which the early development goes 

 on in the two groups. The Sauropsidan egg is large enough 

 to contain within its own confines the amount of yolk neces- 

 sary for the production of a young one complete in all its 

 parts and capable of leading an independent existence 

 immediately it leaves the shell. Furthermore, it is also large 



' In certain Amniotes the layers in question appear also to participate 

 in the formation of the inner lining of the anniion (anniiotic ectoderm) 

 (cf . Asslieton ['09], pp. 248-9), but this does not affect the statement in 

 the text. In the Sauropsida and Monotremata I think I am correct in 

 saying that no sharp distinction is recognisable between the embryonal 

 and extra-embryonal regions of the ectoderm, hence it is difficult, if not 

 impossiljle, to determine Avith certainty their relative participation in 

 the formation of the anniiotic ectoderm. 



