II.] , THE NEURAL TUBE. 37 



body cavity ; it is divided into a thoracic or pleural, and an 

 abdominal or peritoneal cavity; these two cavities are, how- 

 ever., from their mode of origin, portions of one and the same 

 tube. Thus a transverse section of a vertebrate animal 

 always shpws the same fundamental structure: above a single 

 tube, below a double tube, the latter consisting of one tube 

 enclosed within another, the inner being the alimentary 

 canal, the outer the general cavity of the body. Into such 

 a triple tube the simple tubular embryonic sac of the chick 

 is converted by a series of changes of a remarkable character. 



The upper or neural tube is formed in the following way. 

 At a very early period the upper surface of the blastoderm 

 in the region which will become the embryo, is raised up 

 into two ridges or folds which run parallel to each other at a 

 short distance on either side of what will be the long axis of 

 the embryo, and thus leave between them a shallow longitu- 

 dinal groove (Fig. 8, B, also Figs. 11, 12, m.c). As these ridges, 

 which bear the name of medullary folds, increase in height 

 they arch over towards each other and eventually meet 

 and coalesce in the middle line, thus converting the groove 

 into a canal, which at the same time becomes closed at either 

 end (Fig. 8, F, /, also Fig. 13. .M.). The cavity so 

 formed is the cavity of the neural tube, and eventually 

 becomes the cerebro-spinal canal. 



The lower double tube, that of the alimentary canal, and 

 of the general cavity of the body, is formed in an entirely 

 different way. It is, broadly speaking, the result of the junc- 

 tion and coalescence of the fundamental embryonic folds, the 

 head-fold, tail-fold, and lateral folds ; in a certain sense the 

 cavity of the body is the cavity of the tubular sac described 

 in the last paragraph. 



But it is obvious that a tubular sac formed by the folding 

 in of a single sheet of tissue, such as we have hitherto con- 

 sidered the blastoderm to be, must be a simple tubular sac 

 possessing a single cavity only. The blastoderm however 

 does not long remain a single sheet, but speedily becomes a 

 double sheet of such a kind that, when folded in, it gives rise 

 to a double tube. 



Very early the blastoderm becomes thickened in the 

 region of the embryo, the thickening being chiefly due to an 

 increase in the middle layer or mesoblast, while at the same 



