340 HIROTA; OX THE SERO-AMXIOTIC COXXECTIOX, 



the Proamnion. ' The extra-embryonic coelomic cavities \yhich ha\ e 

 thus met in front of the Proamnion, do not fuse together for a time, 

 but have a median mesoblastic septum (M), consisting of two sheets 

 of cells, each limiting the extra-embryonic coelomic cavity of its own 

 side. At this stagne the head fold of the amnion is raised alonof a 

 line which passes through the anterior limit of the proamnion 

 (F. F.). 



When the embryo is 40-45 hours old (Diag. 3), the head is 

 somewhat sunken down in the yolk sac, and the head fold of the 

 amnion has grown posteriorly over the anterior part of the head. Thus 

 the head is received into a cavity which ends blindly in front and is 

 open posteriorly (Diag. 4). The dotted line of Diag. 3 shows the 

 limits of this cavity. The mesoblastic septum is now obliterated near 

 the dnus tenninalis but the backward oTOwth of the amniotic fold has 

 prolonged it in the posterior direction (M). The proamnion (P), 

 into which the extra-embryonic coelomic cavities gradually penetrate 

 from both sides, is now out of sight, concealed beneath the he;id and 

 the amniotic fold. In Fig. 1, and Figs. 57-51), the surface \iew 

 and the sections of the mesoblastic septum are represented. 



In Chelonia the extra-embryonic coelomic cavities secondarily 

 insinuate themselves from both sides into the iimniotic fold, which 

 is at first purely epiblastic (See Figs. 2-4 and Figs. 17-10 of Mitsu- 

 kuri's article). In the chick the extra-embryonic coelomic cavities, 

 which are of course bounded by the mesoblast, spread themselves, from 

 the very beginning of folding, almost coextensively with the epiblast, (jr 

 in other words, nearly up to the posterior horse-shoe sha|)ed edge, so 

 tliat from the initial period, the dorsal part of the amniotic fold of the 

 chick does not contain an extensive area tree from the mesol)last, 



1. E. Ravn : Ueber die Mesodermfreie Stelle in der Keimscheibe des Hükaerembryos. 

 Ai-ch. f. Anat. u. Physiol. (Auat. Abth.). 1886. 



