EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 251 
In reference to the relation between the protochordal plate 
and the secondary growth centre, Hubrecht (p. 58) writes : 
“ The confluence between the earliest ectodermal downgrowth 
with the protochordal plate has up to now not been specially 
examined in reptiles. Still, one may conclude from the figures 
here given, which I copy from other authors, that it comes 
about in exactly the same way as we noticed it in I’arsius for 
Mammals, and in Hypogeophis for Amphibians.” 
Certainly the condition of Hypogeophis is more like the 
reptilian condition than is the condition such as occurs in 
the Annra or Urodela, in which there is from the first an 
opening from the exterior into tlie future archenteron, namely, 
a true blastopore. 
Therefore, in comparing reptilian gut formations with 
others, we ought to compare it more closely with mammals 
and birds, and contrast it with Amphibians such as Anura 
and Urodela, and with fishes and cyclostomes. 
Might it not be well to restrict the term ‘‘archenteron” to 
the part of the gnt which is due solely to protogenetic in- 
fluence, and call the part of the gut (including neurenteric 
canal) which is due to deuterogenetic influence by some such 
term as “ metentei’on ” (vide diagram). If this is a true dis- 
tinction, then we see that there is a very marked difference 
between the Amniotes on the one hand and all the other 
vertebrates. 
In the Amniotes there is an archenteron formed by infil- 
tration of fluid between an upper and anterior wall of cells, 
usually a thin membrane, and a lower or ventral mass of 
either cells or yolk mass, which, when formed, is not in open 
communication with the exterior — that is to say, there is no 
real blastopore. This cavity is, of course, that known as 
blastocyst cavity in mammals, subgerminal cavity in birds 
and reptiles. It is only at a later stage after the growth in 
length has started by the oingin of the deuterogenetic centre 
that a passage is formed which varies very much in its degree 
of development in different types. Thus in birds it is only 
recognisable as a narrow and evanescent canal, the neurenteric 
