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RIOHARD ASSHETOX. 
canal ; in mammals it is usually not more developed, but in a 
few cases it is for a while recognisable as a distinct perfora- 
tion or neurenteric canal, as, for instance, in the human 
embryo (Spee), the hedgehog (Hubrecht), Ornithodelphia 
(Wilson and Hill). But in the reptiles it is so evident as a 
passage leadiug at first into a blind pouch (but later com- 
municating with the subgerminal cavity) that it has been 
mistaken for a true archenteron and true blastopore. 
The true archenteron or subgerminal cavity is not so well 
marked in some (Tropidonotus) as it is in others (Lacerta, 
Platydactylus) ; but in all the “ metenteron ” is obvious, and 
subsequently a perforation occurs, and communication 
between the protogenetic cavity (the subgenuinal cavity 
or archenteron) and the deuterogenetic cavity (neurenteric 
canal or metenteron) is established. 
And I do not think this is a subtle distinction only, but 
one Avhich is essential to the understanding of the true way 
in which the vertebrate embryo grows, and to my mind it 
tends towards simplification. 
If it were not for the hypothetical vermactinian ancestor I 
see no reason why Hubrecht’s and my own views should not 
absolutely coincide. For Hubrecht quite agrees with the 
view that this so-called invagination cavity of reptiles, this 
cavity slit or porus,” is not archenteron, and I believe that 
if he would himself perform operations on developing 
Amphibian, Avine, or Teleosteau eggs, he would convince 
himself that the relations of the protogenetic and deutero- 
genetic centres are morphologically and pln’siologically as I 
have several times indicated them to be. 
Gastrulation in the Ornithodelphia. 
With reference to the description given by Wilson and 
Hill (’07) of the corresponding stages in Ornithorhynchns, I 
should like to make one suggestion. 
Hubrecht accepts their interpretations, and regards the 
curious mass situated some distance in front of the primitive 
