1611 
yolksack which forms the anterior (resp. superior) wall of the liver- 
invagination, appears as a distinct organ, a crescentic area which 
I bave called in connection with its topographic situation : praecere- 
bral or terminal entodermarea. [ here prefer using the term proam- 
nion-area anticipating by this name its significance in Amniota. 
The fact that the proamnion-area remains for a long time free 
from mesoderm is probably caused by the peculiar allogenetic growing 
circumstances which prevail in the development of the yolkmesoderm, 
but it may be due to an autogenetic, morphological cause unknown 
to me. We have already mentioned the separate centre of proli- 
feration of the yolk-mesoderm. This so-called ventral mesoderm 
arises in a slightly different way from the true somatic mesoderm) 
Now the fore-side of the yolk receives its mesodermal covering 
for the greater part from the ventral mesoderm, which arises on 
the hind side and for the lesser part from the cephalic mesoderm 
resp. from the pericard. The rapid outgrowth of the head region 
consuming a great mass of the cephalic mesodermal material, the 
ventral mesoderm being obliged to grow over the whole yolksack 
before reaching the fore-side, the consequence is that this side of 
the yolk will remain for a long time without mesodermal covering. 
What however may have been the real cause of the appearance 
of the proamnion-area, it may suffice here to state the presente 
of this organ in primitive Amphibians. 
Now let us try to retrace the changes, which the amphibian 
ovum undergoes if the eggs are laid outside the water and the 
free larval state is consequently abbreviated or wholly abolished. 
It may happen in this case that a more or less intense, internal 
nursing is provided, the embryo remaining during its whole 
development or during part of it in protective cutaneous folds or 
pits of the female (Hyla goeldi Boul., Nototrema, Pipa americana 
Laur., Blueophorus reticulatus Günth. ete.), in the buecal cavity or 
in the oral saes of the male (Rhinoderma) or in the genital ducts 
of the female (Salamandra, Spelerpes, Typhlonectes, Dermophis 
thomensis Boe. etc). In the second place the parents may surround 
the synoion with a mass of froth, which dries up to a horny sheet 
1) The ventral mesoderm does not arise as an epiblastie ingrowth like the 
somatic mesoderm (dorsal plate), but it proliferates from the ventral border of the 
blastopore (somatopore), where yolkcells pass into the epiblastic ones, grows in 
between the yolk and its epiblastcovering, and increases probably partly by 
delamination of peripheral yolkcells. It never divides into segments. Its character 
and development therefore recall that of the protochordal plate and that of the 
anterior head-mesoderm or speaking generally that of the cephalic mesoderm. | 
shall revert later on to its great importance in the mammalian fruitbladder. 
