22 .GENERAL EMBRYOLOGY. 
embryonic region. It is separated from the proamniotic area by a bar of mesoderm 
in which the pericardial cavity afterwards appears. The anterior part of this 
second area is situated in 
front of the neural groove, 
and as, at a later period, it 
forms a septum between the 
ae primitive mouth and_ the 
area 
/, Pevieardiay PHarynx, it may be termed 
__ Bueco- ( Hy] area the bucco-pharyngeal area. 
merece AW The posterior part of the 
| \ AMT Amniotic grea forms the floor of the 
Hl 
{| 
area 
medullary groove, and as 
| the notochord is formed 
pi] Fiacental from its entodermal layer 
i] we have named it the noto- 
chordal area. 
Chorionic The third area corresponds 
a to the posterior part of the 
primitive streak. It ex- 
tends from the base of the 
tail towards the umbilicus, 
SURFACE AREAS OF THE BLASTODERM. forming the cloacal mem- 
brane, which forms the 
posterior boundary of the primitive alimentary canal. It is eventually perforated 
by the genito-urinary and anal apertures. 
Except in the areas just described, the blastodermic membrane is trilaminar, 
and at an early period it is possible to distinguish the regions in which the heart 
and pericardium, the amnion and the placental and non-placental parts of the 
4 
' | 
a 
| 
i 
Notochordal 
area 
reel: 
Embryonic area € 
a eee ty ATNIOLIC 
| NG | area 
I | 
“— Placental area 
' Chorionic area 
an a 
EC Notochordal area 
Pericardial EB) | Primitive streak 
| | 
Amniotie area 
II 
_U Placental area 
Ss 
~yChorionic area 
Proamniotic 
area 
Bueco-pharyngeal 
area 
Fic. 16.—SECTIONS SHOWING THE DIFFERENT AREAS OF THE BLASTODERMIC VESICLE (Diagrammatic). 
I. Transverse section ; II. Longitudinal section, 
EC. Ectoderm. M. Mesoderm. N. Notochordal thickening. 
EN. Entoderm. NG. Neural groove. 
chorion are subsequently developed. These regions form fairly well-defined areas, 
to the relative positions of which reference may now be made. 
The anterior part of the embryonic area in front of the bucco-pharyngeal area 
is the region in which the pericardium and heart are developed, and it may therefore 
be termed the pericardial area. 
The blastodermic membrane immediately surrounding the embryonic area, 
