12 INTRODUCTION. 
the tube results (fig. 6, 2c), the cavity of which persists throughout life 
as the cavities (ventricles) of the brain and the central canal of the 
spinal cord. From the cells of the walls of the canal the nervous tissue 
arises. 
This process of infolding progresses from in front backward. For 
a time, in some vertebrates, a small opening, the anterior neuropore, 
persists at the anterior end. The infolding extends back to the poste- 
rior end of the neural plate so that, as will readily be understood, the 
whole limits of the blastopore are included in the floor of the neural 
canal. Occasionally the closure of the neural folds is completed before 
that of the blastopore so that for a short time a short tube, the neuren- 
teric canal (fig. 7), connects the archenteron with the neural canal. 
Soon after the closure of the neural tube the fused tissue splits horizont- 
ally, separating the nervous sys- 
tem from the rest of the ectoderm. 
SST ee 
[AC] SURE EPR Per EE eo mIorl Bejan Z 
ss iairareanat mee Its subsequent history will be 
aa TM od int 
efelefofofoe 
Qe, 
traced in the section of the Ner- 
¥ vous System. : 
he. The Notochord.—Immediately 
beneath the neural plate is an axial 
strip of entoderm (fig. 5, ch), 
Fic. 7.—Schematic section of the hinder bounded on either side of the out- 
end of an amphibian embryo, showing the growing mesothelium. When the 
relations of the neurenteric canal. ac, alimen- . 
tary canal; ec, ectoderm (black); , notochord; latter separates (p. 10) this band 
sa aaa aa rics ag ?, is momentarily rejoined to the rest 
of the entoderm butis still recogniz- 
able from its different cells. It soon rolls into arod (a tube in some 
amphibians and birds), is cut off from the rest (fig. 6, m) and lies 
between the digestive tract and the nervous system where it forms an 
axis around which the skull and vertebral column develop later. 
The Digestive Tract.—After the separation of the notochord, the 
entoderm forms a tube, closed in front and usually behind as well. 
The anterior end of the tube abuts against the ectoderm of the ventral 
side of the embryo. Later the ectoderm grows in at the point of con- 
tact, carrying the entoderm before it and forming a pocket, the stomo- 
deum, which gives rise to the cavity of the mouth. (In some the 
stomodeal ingrowth is at first solid, the pocket being formed later by 
splitting). Eventually the ectoderm and entoderm fuse at the bottom 
of the cup, and then the fused area breaks through, placing the archen- 
