THE NOTOCHORD. 23 
including the proamniotic part in those animals in which it exists, is the amniotic 
area, and this is bounded externally by a band of elevated and thickened ectoderm, 
which indicates the placental area. The latter, together with the blastoderm over 
the rest of the ovum, forms the chorionic area, which is separable, therefore, into 
placental and non-placental portions. 
These areas are further referred to in the description of the folding off of the 
embryo and the formation of the foetal membranes and placenta. 
Formation of the Notochord.—The notochord is the primitive skeletal axis of 
the embryo. When differentiated it forms a rod which intervenes between the 
ectodermal neural tube and the entoderm of the primitive alimentary canal. It is 
developed from the entoderm beneath the neural groove in the notochordal area. 
A linear strip of entoderm thickens and then separates as a solid rod of cells, the 
continuity of the entodermal layer being restored beneath it. When it is completed 
the notochord extends from a point immediately behind the primitive forebrain, and 
beneath the anterior end of the midbrain, to the anterior end of the primitive streak, 
and in later stages, as the skeleton is formed, the notochord can be traced from the 
post-sphenoid section of the base of the skull, which is situated beneath the mid- 
brain, to the tip of the coccyx. 
The separation of the notochord from the entoderm commences in the cervical 
region, and extends forwards and backwards. The anterior extremity is the last part 
to be detached, the separation occurring shortly after the perforation and disappear- 
ance of the bucco-pharyngeal membrane. 
The cellular notochord develops a cuticular sheath ; it is subsequently surrounded 
by mesoderm which separates it both from the neural tube and the entoderm, 
C 
Fic. 17.—ExtTENSION OF MESODERM AND FORMATION OF Ca@LOM (Diagrammatic). 
A. Mesoderm spreading from the sides of the ectodermal primitive streak, and extending between the ectoderm 
and entoderm. B. Further extension of the mesoderm and appearance of ccelomic cleft-like spaces. 
C. Complete delamination of the mesoderm and formation of ccelom. 
BC, Blastodermic cavity. C. Ccelom. EC. Ectoderm. EN, Entoderm. M. Mesoderm. 
P. Primitive Streak. SoP. Somatopleure. SpP. Splanchnopleure. 
and which is ultimately transformed into the vertebree and their ligaments, the 
intervertebral discs, the basi-sphenoid and basi-occipital parts of the skull, and the 
membranes of the brain and cord. 
As the surrounding mesoderm is cifferentiated the notochord becomes nodulated ; 
