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definitive amnion ') develops immediately by outgrowth of mesoderm- 
containing folds. 
The great merit of Husrrcur’s theoretical considerations in this 
question is to be sought partly in the searching criticism exer- 
cised on established ideas e.g. on the axiom that amnion-develop- 
ment is eo ipse more primitive in Sauropsida than in Mammalia, partly 
in the widening of our mental horizon by opening new and unex- 
pected perspectives which incite to new investigations. Therefore I 
feel obliged in honour of the memory of the great Dutch embryolo- 
gist not to shrink from equitable criticism or from necessary correc- 
tion and supplementation of his fertile ideas. 
The premise mentioned sub 1 would be correct if the amnion- 
cavity really served in young stages as a protective watercushion. 
But this is by no means the case. In young mammalian foetera 
protection is furnished by what I will call here provisionally 
the trophoblast-bladder and the primitive amnion (arehamnion 
Resink) is only a means of pushing the embryonic knob into this 
protective watercushion. Therefore | am inclined with RestnK *) 
to consider amnion-formation by entypy as primitive, whether this 
appears as a hollow invagination or as a solid ingrowth. I will 
revert to this point later on. 
In a second question. the significance of the trophoblast, I also 
have a somewhat divergent opinion. L will not deny the important 
part played by this embryonic appendage in the amniogenesis of 
all Amniotes, but I am not convinced that it is already present as 
a larval envelope in Anamnia, or that it is represented by the 
covering layer of the epidermis (Deckschicht) of amphibian larves. 
It is true that Hvsrecur has altered his opinion on this point in his 
later publications to a certain extent, but nevertheless he proposes 
to divide the Vertebrates in Achoria (Cyclostomata and Selachii, 
Vertebrates without trophoblast) and Choriata (all the rest, possessing 
this larval envelope) *). In my opinion therefore | am fully justified 
in making some critical remarks on this point. 
Of course the trophoblast of Mammals must have arisen from 
the ectodermal covering of anamniote larvae, one therefore is 
justified in saying that it is present potentially in the larval 
1) 1 use the term amnion in this case in a wide sense i.e. amnion s.s. + chorion 
resp. serous membrane. 
2 A. J. Resinx. Die Stammentwicklung der embryonalen Organe. Tijdschr. 
Nederl. Dierk. Ver. D. VIII, afl. 3, 1903. 
3) A. A. W. Huprecur. Ontogenetic phenomena in Mammals etc. Quart. Journ. 
Mierose. Sci. Vol. 53, 1908, p. 81—83. 
