12 



INTRODUCTION. 



the tube results (fig. 6, nc}, 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. 

 Its subsequent history will be 

 traced in the section of the Ner- 

 vous System. 



The Notochord. Immediately 

 beneath the neural plate is an axial 

 strip of entoderm (fig. 5, ch}, 



FIG. 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) ; n, notochord ; latter separates (p. IO) this band 



ne. neurenteric canal; nt, neural tube; p, j s momentarily rejoined to the rest 

 proctodeum; pa, post-anal gut; y, yolk. J 



of the entoderm but is still recogniz- 

 able from its different cells. It soon rolls into a rod (a tube in some 

 amphibians and birds), is cut off from the rest (fig. 6, n] 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- 



