GROWTH OF THE BLASTOPBEM. 



437 



sides of wliioli, the medullary folds (mf), grow up and meet, 

 forming a tube, the neural tube, the future cerebro-spinal canal 

 (fig. 232, Hb). Below this, during the first day, appears pre- 

 viously a rod of cells, the future notochord (Not). 



The blastoderm early in development becomes thickened by 

 the growth of the mesoblast (Mes). The mesoblast on each 

 side then cleaves into two layers. Tlie upper part of the 

 mesoblast unites with the epiblast to form an outer layer, the 

 lower with the hypoblast to form a lower and inner layer. 

 These two layers, known respectively as the upper, outer, or 

 somatopleure, and the lower, inner, or splanchnopleure, grow 

 down and meet to form two tubes. The inner tube, lined 



Mo I 



Fig. 231. — Transverse Section or Blastoderm, inculjated for eighteen 

 hours, (After Foster and Balfour.) 



Ep, Epiblast; Mes, mesoblast; Hy, hypoblast; 7ftf7, medullary groove ; m/, medul- 

 lary fold ; Not, notochord, 



by hypoblast, is the alimentary canal, wnich in time becomes 

 perforated at each end by two infoldings of the epiblast, form- 

 ing the mouth and anus. The outer tube forms the tube of 

 the body, the space between these two tubes being the body 

 cavity. The embryonic sac is connected with the yolk-sac by a 

 gradually narrowing hollow stalk. This stalk, like the embryonic 

 sac, is double, there being a somatic and a splanchnic tube : the 

 splanchnic tube connects the alimentary canal with the cavity 

 of the yolk-sac ; the somatic stalk connects the body walls of 

 the embryo with the somatopleure of the yolk-sac. Very soon 

 in development the splanchnic canal becomes obliterated, and 

 thus shuts ofi^ the yolk from entering the alimentary tube. The 



