EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 15 
a dorsal mouthslit, a “ Riickenmund” (Fig. 160) of a 
vermactinial stage of development. 
The mammalian blastopore, rudimentary, rare, and eva- 
nescent as it is, still reminds us of the blastopore*of the 
invertebrates in this respect that in its immediate vicinity those 
cell proliferations commence which lead up to the formation 
of the so-called mesodermal structures. 
~ 
5. Theoretical Speculations about the Origin of 
the Trophoblast. 
The facts with which we have up to now become acquainted 
concerning the early development of didelphic and mono- 
delphic mammals (the so-called marsupials and the placental 
mammalia) fully justify the conclusion that the embryo 
already in its very earliest ontogenetic phases is provided 
with a larval envelope, an ‘ Embryonalhiille.” ‘To this layer 
of cells we have given the name of trophoblast. Later on 
we shall see that this layer, though it is at the outset only 
one cell thick, can undergo the most varied proliferations in 
very divergent spots, and that such proliferations are at the 
basis of the whole phenomenon of placentation. The fact 
that to these proliferations and their significance for the 
early nutrition of the embryo, attention was first directed 
(Hubrecht, ’88, ’89) before the more general significance of 
the layer as a larval envelope had yet been fully appreciated 
was the cause that the name of trophoblast has been given 
to it. We will return to this when the phenomena of 
placentation will be discussed. 
It cannot be denied that the consequences of considering 
the trophoblast as a larval envelope and of introducing this 
1 It remains to be seen whether the name of “ trophoderm,” introduced by 
Sedgwick Minot (03) for that portion of the trophoblast which takes an 
active part in placentation, is a desirable innovation, or rather a synonymous 
encumbrance. But even in the former case Duval’s proposal of the name of 
* ecto-placenta ” has the priority. 
