512 
mesoblast has throughout split into two layers, the coelomic cavity 
has appeared not only in the embryo proper but in the extra-embry- 
onic part of the blastoderm. A fold of the somatic mesoblast enclos- 
ing a part of the coelomic cavity now insinuates itself on each side 
into the hitherto solid epiblastic amniotic fold of the dorsal region, 
dividing the latter into two limbs: the inner amnion proper and the 
outer serous envelope. In the mid-dorsal line, the two mesoblastic 
folds almost come together but do not touch each other as a string 
of cells connects the serous envelope and the amnion, and separates 
the extra-embryonic coelomic cavities of the two moieties of amnion 
(Diag. III‘). In the head region, the coelomic cavities become early 
united across (Diag. III). 
6. One of the most important results of the present investigation 
is in reference to this connection between the amnion and the serous 
envelope. Contrary to what is hitherto known, the extra- 
embryonic coelomic cavities of the two moieties of the 
amnion are never united across with each other over 
the dorsal region of the embryo. A connection — quite 
elongated and definite in later stages — between the 
amnion and the serous envelope separates them to the 
very end of the development. This structure causes 
great peculiarities in the foetal membranes in later 
stages. This connection I propose to call the sero- 
amniotic connection. It does not extend into the region of the 
sunken head where the amnion originally consisted of the hypoblast 
and epiblast. 
7. The most remarkable part in the history of the amnion is 
the following fact: the anterior and lateral ~folds which starting 
from the head have gradually extended backward over the whole em- 
bryo, do not stop at the posterior end of the embryo 
but continue to grow backward, although diminished 
in its width, until finally there is produced a tube 
