DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 437 
part of the embryonic area, and the horns of the crescent 
advance, passing forward a short distance from the outer mar- 
gins of the embryonic area until they have passed its anterior 
extremity; then they converge and fuse in front of the pro- 
amnion, as in the rabbit. The posterior portions of the inner 
margins of the ring-like space which is thus formed pass inwards 
into the embryonic region, dividing the embryonic mesoblast 
into somatic and splanchnic layers (fig. 16 G, Pl. XXVI); 
but the anterior portions of the inner margins of the ring never 
penetrate the mesoblast which surrounds the bucco-pharyngeal 
membrane and lies at the sides of the anterior portion of the 
notochord, and which constitutes the ‘‘ Herzanlage ” of Hensen 
(18) or the “ pericephalic mesoblast” of Fleischmann (12). This 
mesoblast has the same horseshoe-shaped outline in the mouse 
at the ninth day before protovertebral somites have appeared 
that it has in the rabbit with two protovertebral somites (18). 
In the rabbit the horseshoe-shaped mass of mesoblast is pene- 
trated by the ccelom before the cornua of that space have met 
in the mesoblast in front of the pro-amnion (3, pl. xxiv, figs. 2 
and 3). In the rat and the mouse the pericephalic ccelom is 
formed by a forward extension of the embryonic ccelom into 
the pericephalic mesoblast ; this extension takes place simul- 
taneously on both sides, and the two halves meet in the middle 
line in front of the bucco-pharyngeal membrane in the latter 
part of the ninth day (fig. 19 4, PC) some hours after the 
fusion of the horns of the extra-embryonic celom in front of 
the pro-amnion. In relation, however, to the general develop- 
ment of the embryo, the completion of the pericephalic celom 
in the rat and the mouse occurs about the same time as in the 
rabbit ; that is, at the period when three protovertebral somites 
have appeared. 
The pericephalic ceelom of the sheep, the rabbit, the rat, 
and the mouse becomes developed into the pericardial cavity ; 
whilst in the cat, according to Fleischmann (12), it disappears. 
Fleischmann’s plates, however, do not substantiate his state- 
ments ; on the contrary, they show that instead of disappearing 
the pericephalic celom of the cat becomes expanded, and takes 
VOL. XXXII, PART 111.—NEW SER. GG 
