1605 
breathing, even in pelagic forms which never come ashore, this 
adaptation may be recorded as a secondary one. Thus we safely 
can suppose, as has been done by the majority of investigators, that 
the appearance of primary foetal membranes is to be considered as 
an adaptation to terrestrial life, an adaptation which makes it 
possible, that embryonic development takes place in liquid surround- 
ings, though outside the water’). 
Now the question is whether oviparity in Sauropsida may be 
deduced directly from oviparity in Amphibians, the holoblastic 
cleavage-type of the last group being changed simultaneously by 
accumulation of yolk in the meroblastie one of the first, the yolk- 
poverty of Mammalian eggs consequently being secondary, or whether 
the evolution has proceeded in a reverse direction. Most of the 
older and some modern investigators have declared in favour of the 
first alternative. According to them the mode of development of 
foetal membranes by folding up, so typical for the Sauropsidan egg 
is to be considered as primitive and the various modes of amnion- 
formation in Mammals are to be derived from this one. It is not 
my intention bere to go into all the abortive attempts to explain the 
phylogenetic origin of the amnion, published before the year 1895, 
since Husrrcur has criticized them sufficiently in his well known 
treatise on this subjeet*), but it will suffice to summarize the con- 
ception of SeLenka*), which combines the two older explanations. 
SELENKA believes the amnion to be composed of two originally 
independent folds, growing together by chance, thereby closing the 
amnion-cavily and separating the embryo from its surroundings. 
These two folds are the headfold and the tailfold. The precocious 
segregation of brain and head, and the curving of these organs 
caused by this circumstance, push the head into the yolk, and in 
this way the headfold arises passively. It usually is at first a pro- 
amnion Le. it consists of ectoderm and. entoderm, the last being 
1) | wish without delay to lay stress upon the fact that in my opinion the 
importance of the development within an amnion-cavity for terrestrial forms chiefly 
lies in the circumstance, that in this way the developing embryo is exposed on all 
sides to uniform pressure, while most writers think of protection against mechanical 
injuries from the outside. Tertiary e g-coverings, already abundantly present in 
Amphibians, supply sufficient protection against these injuries. If mechanical 
pressure or shock can penetrate a bird’s eggshell and glaircovering, they certainly 
will be able to pierce the thin chorion and amnion. 
2) A. A. W. Huerecur. Die Phylogenese des Amnions und die Bedeutung des 
Trophoblastes. Verhand. Kon. Akad. Wetensch. Amsterdam. 2e Sectie, D. IV, N. 5. 
5) Vide E. Sererka. Studien zur Entwicklungsgeschichte der Tiere. Bd. 1, Heft 5, 
1892: Zur Entstehungsgeschichte des Amnions. p. 186—189. 
103* 
