290 A. A. W. HUBREOHT. 
mesoblast. After the formation of this third embryonic layer 
has commenced the separation of somatic and splanchnic 
mesoblast can very soon be distinguished. The former follows 
the contours of the epiblastic disc, and folds upwards for the 
formation of the mesoblastic amnion fold all along the circular 
insertion of this disc that was above mentioned. The epi- 
blastic fold of the amnion is thus not a double one as it is in 
all diagrams of amniogenesis. But it is a single sheet of 
epiblast cells that grows upwards simultaneously with the 
bending upwards of the double somatic fold. Finally, an 
amnion navel is formed, and when this has vanished and the 
amnion has become a closed sac all above the embryo (at this 
time the allantois just commences to protrude), the similarity 
with the other Amniota is again established. This apparently 
strange mode of development of the amnion is easily under- 
stood when we remember that after the splitting off of the 
embryonic epiblast the space that remains between this layer 
and the outer epiblast must needs become the cavity where 
the amnion will develop. A layer of external epiblast for the 
expected “serous envelope of v. Ber” (which arises simul- 
taneously with the amnion) is, however, already present in 
consequence of the splitting phenomenon so often referred to. 
And so the amniogenesis is actually restricted to the formation 
of a double fold of somatic mesoblast and to an extension up- 
wards of the circle of insertion of the embryonic disc. The 
phenomenon will be still better understood after comparison 
of the diagram 32 with the figures 51 and 52, Pl. XXV. The 
formation of the amnion takes place more or less simul- 
taneously all around the circumference of the embryonic disc ; 
the point of definite closure lies above the hinder portion of 
the trunk. The head is at an early moment excluded from 
the true amniotic fold by the fact of its being bent downwards 
and reaching into the yolk-sac, enclosed by a didermic sheath 
which forms a so-called pro-amnion, about which I will give 
more details in a later paper. Long after the true amnion has 
been quite completed the head gradually emerges from this 
pro-amniotic pit. The amnion has then gradually increased in 
