444. ARTHUR ROBINSON. 
lizard, and the bird, when the hood is first formed, it covers 
only the posterior portion of the embryonic area, and its ante- 
rior margin abuts against the anterior margin of the pro-amnion 
fold which covers the cephalic portion of the embryo. As the 
pro-amnion fold atrophies the hood-like caudal amnion extends 
until the disappearance of the pro-amnion, when it completely 
encloses the embryonic area. 
In the rat and the mouse, therefore, amnion formation follows 
closely in the path marked out by mesoblast formation ; it 
commences posteriorly, extends laterally, and is completed 
anteriorly. In the majority of other Amniota the extension of 
the amnion takes place in a similar manner; such differences 
as are met with are due to individual variations from a common 
plan. 
So far as I am aware the general plan of amnion formation 
is modified to an important extent only in the human ovum 
and the ovum of the guinea-pig; for in the sheep, although 
the celom apparently commences as two distinct portions, the 
amnion folds do not rise until it has become one continuous 
cavity ; and Bonnet says that upon its first appearance the 
amnion fold is not separable into distinct sections, but forms a 
continuous ridge round the embryonic area (6); whilst in the 
Chelonians (34) the peculiarities are due to interference with 
the completion of the process, not to any alteration of the 
plan. 
The guinea-pig has been previously referred to, and now the 
peculiarities of the human ovum may be considered. 
According to the typical schemata of Kolliker (26), there is 
a period during which the embryo with its yolk-sac and 
amnion lie free within the serous sheath, to which they after- 
wards become united by means of the allantois. His (21) has 
shown that this period is entirely supposititious so far as the 
human ovum is concerned; and he has pointed out that 
through the whole period of intra-uterine life the embryo never 
loses its connection with the chorion. He terms the stalk of 
connection the “ Bauchstiel,” and looks upon it as the direct 
backward prolongation of the embryo, the allantois taking 
