1615 
just as commonly happens with other embryonic respiratory organs 
of Amphibians. The differentiation of the amphibian embryo or 
larva — built up of more or less equivalent cells — into a small 
embryonic shield forming exclusively the young animal and a large 
exembryonie fruitbladder, being shed at birth, may be sufficiently 
explained in this way. 
This situation once obtained, it will suffice for the protection of 
the small embryonic area to invaginate the latter into the large 
trophoblastbladder and in this manner a fruitbladder with entyped 
embryonic area has arisen. As the proamnion area is situated at 
the foreside, invagination of this part of the fruitbladder does not 
reduce the respiratory surface and one can understand that this 
process will take place chiefly at this side, setting aside the con- 
sideration that the precocious development of the headregion may 
serve as an ontogenetic (not as a phylogenetic) causal moment for 
this phenomenon. '). 
At the backside the entypy will take place exactly at the spot 
where the ventral mesoderm grows out i.e. at the spot where from 
the beginning the connection between embryonic and exembryonic 
mesoblast and between embryonic and exembryonic bloodvessel- 
systems are present. Now entypy withdraws at this point a part of 
the respiratory diplotrophoblast from the surface of the fruitbladder, 
but on the other side the connection between embryonic and ex- 
embryonic vesselsystem is thus maintained in a very intimate way. 
I think that in this manner the peculiar circumstance that the con- 
nective stalk (Haftstiel) of so many mammalian germbladders (e.g. 
in Primates) is always found at the caudal side of the embryo is 
conclusively explained. I therefore cannot agree with ResiNK, who 
supposes the amnionstalk to be originally uniformly vascularized and 
1 think I have demonstrated that important arguments can be brought 
forward for considering the appearance of the connective stalk at 
the caudal side of the embryo and at the back of the archamnion 
as a primitive characteristic. 
1) Huprecut considers the proamnion a formation without morphological signi- 
ficance (see p. 79. 1908, note) and ascribes the lack of mesoderm and bloodvessels 
to the rapid growth of the headregion. His arguments on this point are however 
somewhat contradictory and the fact that the proamnion is present in nearly all 
groups of Amniota and is especially well developed in archaic Sauropsida 
(Sphenodon, Chelonia) makes it rather improbable that this organ should not 
possess an important morphological signification. Moreover its presence in Amphibia 
strengthens my opinion in this matter. Thus I consider strong development of the 
proamnion as a primitive feature. In Sauropsida it represents the last remainder 
of the original entypy. 
