834 



DEVELOPMENT 



[CH. LIX. 



to fold off from the rest of the ovum. A sulcus appears all round 

 the margins of the area, and over this sulcus the area bends forwards, 

 backwards, and laterally. It looks as if some constricting agent had 

 been placed round the margin of the area, and that afterwards the 

 area above the constriction, and the area below had gone on growing 

 rapidly. In this way, the ovum is clearly separated into two parts, 

 an upper, the embryo, and a lower, which becomes the appendages 

 of the embryo. The anterior part of the folded embryonic area is 

 known as the head fold, the posterior as the tail fold, and the two are 

 connected together on each side by the lateral folds. As the constric- 

 tion between the embryonic and non-embryonic parts affects the 



FIG. 630. Diagram of a transverse section 

 through a mammalian ovum at the period 

 when the folding off of the embryo has 

 commenced. 1, Neural tube ; 2, proto- 

 vertebral somite ; 8, epiblast ; 4, somatic 

 mesoblast ; 5, splanchnic mesoblast ; 6, 

 hypoblast ; 7, notochord ; 8, primitive 

 alimentary canal ; 9, coelom ; 10, vitello- 

 intestinal duct; 11, yolk sac ; 12, lateral 

 fold of amnion. 



FIG. 631. Diagram of a longitudinal section of a 

 mammalian ovum at the period when the folding ofl' 

 of the embryo has commenced. 1, Neural tube; 

 J, epiblast ; 3, notochord ; 4, stomadceal space ; 5, 

 head fold of amnion ; 6, tail fold of amuion ; 7, 

 hypoblast ; 8, somatic mesoblast ; 9, splanchnic 

 mesoblast ; 10, yolk sac ; 11, coelom ; 12, allantois ; 

 13, hind-gut ; 14, mid-gut ; 15, fore-gut ; 16, peri- 

 cardium. 



interior as well as the exterior of the ovum, it follows that three 

 cavities are present in the embryo. (1) The central canal of the neural 

 tube, which is of course lined by epiblast, (2) A portion of the 

 archenteron lined by hypoblast. (3) A portion of the coelom or 

 cavity of the mesoblast (fig. 630). 



The central canal of the neural tube, as before stated, becomes the 

 cavity of the permanent central nervous system, and it forms the 

 central canal of the spinal cord, the lateral, third and fourth ventricles, 

 and the aqueduct of Sylvius which connects the third and fourth 

 ventricles together. 



The portion of the archenteron enclosed in the embryo forms the 

 primitive gut. The part contained in the head fold is the fore-gut, 



