62 



THE SECOND DAY. 



[CHAP. 



the sides. One of the results of this doubling up of the blasto- 

 derm to form the head is the appearance, below the anterior 

 extremity of the medullary tube, of a short canal, ending 

 blindly in front, but open widely behind (Fig. 16, D], a cut de 

 sac in fact, lined with hypoblast reaching from the extreme 

 front of the embryo to the point where the splanchnopleuric 

 leaf of the head-fold (Fig. 16, F. Sp) turns back on itself. 

 This cul de sac, which of course becomes longer and longer 

 the farther ba.ck the head-fold is carried, is the rudiment of 

 the front end of the alimentary canal, the foregut, as it might 

 be called. In transverse section it appears to be flattened 

 horizontally, and also bent, so as to have its convex surface 

 looking downwards, (Fig. 18 al). At first the anterior end is 

 quite blind, there being no mouth at all ; the formation of 

 this at a subsequent date will be described later on. 



At the end of the first half of the second day the head- 



FIG. 1 6. 



N.C. 



F.So. 



DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE Axis OF AN EMBRYO. 

 The section is supposed to be made at a time when the head -fold has oom^ 

 menced but the tail-fold has not yet appeared. 



N. C. neural canal, closed in front but as yet open behind. Ck. notochord, not 

 reaching to the extreme front, and not as yet fully formed behind. The 

 section being taken in the middle line, the protovertebrse are of course not 

 shewn. In front of the notochord is seen a mass of un cleft mesoblast, 

 which will eventually form part of the skull. D. the commencing fore- 

 gut or front part of the alimentary canal. F. So. Somatopleure, raised 

 up in its peripheral portion into the amniotic fold Am. Sp. Splanchno- 

 pleure. At Sp. it forms the under wall of the foregut; at F. Sp. it is turning 

 round and about to run forward. Just at its turning point the cavity of the 

 heart lit is being developed in its mesoblast. pp. pleuroperitoneal cavity. 

 A epiblast, B mesoblast, C hypoblast, indicated in the rest of the figure by 

 differences in the shading. At the part where these three lines of reference 

 end the mesoblast is as yet uncleft. 



