40 PKELIMINARY ACCOUNT. [CHAP. 



blastoderm or epiblast 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 longitudinal groove 

 (Fig. 9, B, also Figs. 21, 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. 34. Me.}. 

 The cavity so formed is the cavity of the neural tube, 

 and eventually becomes the cerebro-spinal canal. Its 

 walls are wholly formed of epiblast. 



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 junction and coalescence of the funda- 

 mental 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 considered 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 



