438 ARTHUR ROBINSON. 
part in the formation of the pericardial cavity just as in other 
animals. 
The Amnion. 
If we accept Selenka’s dictum that amnion formation is 
essentially the production of a hood for the embryo (46, 
p- 181), we are bound to admit that the amnion may con- 
sist of only one layer, the epiblast alone, as in the guinea- 
pig, or of two layers. Usually two layers are found in an 
amnion fold, and they may be either epiblast and mesoblast, 
as in the tail amnion fold of the rabbit, or epiblast and hypo- 
blast, as in the cephalic amnion fold of the opossum. In 
making this admission, however, we accept the conclusion 
that amnion formation may take place in entirely different 
ways in different animals, for in all the Amniota except the 
guinea-pig (45) the amnion folds are formed by two layers: 
the caudal fold always by somatic mesoblast and epiblast, the 
cephalic fold in most, if not all cases, first by epiblast and 
hypoblast, which constitute the so-called pro-amnion, and after- 
wards by somatic mesoblast and epiblast. In the majority of 
the Amniota a disc of epiblast rests upon the upper pole of 
the ovum. As the amnion folds rise the margin of the epi- 
blastic dise is gradually contracted, and it disappears finally 
when its remaining portions fuse in the situation of the amnion 
navel. In the guinea-pig, as in the rat and the mouse, the 
epiblast is not formed as a flat disc with outspreading margins, 
but as a solid mass of cells within which a cavity appears 
(fig.11, Pl. XXIII). In each of the above-mentioned animals the 
cavity is at first enclosed on all sides by the epiblast ; but in 
the rat and the mouse, by the separation of the margins of 
the epiblast, the cavity is soon opened out, becoming continuous 
with a cavity in the trophoblast (fig. 18, Pl. XXIII), and the two 
together correspond to the extra-embryonic space, which in 
the case of the rabbit is enclosed by the fusion of the placental 
ridge of epiblast with the uterine mucous membrane. After- 
wards, when the mesoblast appears, the margins of the 
epiblast are again carried over the dorsal aspect of the embryo 
at the apices of the amnion folds until tliey meet and fuse at 
the amnion navel. In the guinea-pig the margins of the 
