22 THE HUMAN BODY. 



consists of three cell layers; an outer or epiUast, a middle 

 or mesoblast, and an inner or hypoblast. From this simple 

 sac, presenting no resemblance not merely to a human being 

 but to a vertebrate animal, the foetus is built up by cell 

 division and modification. \ The general history of intra- 

 uterine development" IsKQiucJi too long and too complex to 

 enter upon here, but the formation of certain structures 

 lost at or before birth, and associated with the protection and 

 nourishment of the embryo, may be attempted: they are the 

 yelk sac, the amnion, and the allantois. The developing 

 blastoderm especially thickens in the neighborhood of the 

 embryonal disk and there the outlines of the Body are first 

 laid down. Along the thickening a groove appears, which 



FIG. 166. A, an early blastoderm with the first traces of the primitive groovy 

 B, the same a little later ; /, primitive groove ; d, thickened region of the blas- 

 toderm which directly builds up the embryo. 



marks out the future longitudinal axis and dorsal side of the 

 body, (A, Fig. 166). This groove elongates, its edges rise (B), 

 and finally arch over, meet, and fuse together above it. The 

 tube thus closed in is the rudiment of the cerebro-spinal 

 axis; from its lining epiblastic cells the brain and spinal cord 

 are developed, and its cavity remains throughout life as the 

 central canal of the spinal cord and as the cerebral ventricles, 

 except the fifth (p. 1 65). Some distance on each side of this 

 dorsal tube the mesoblast splits into an outer leaf, adherent 

 to the epiblast, and an inner adherent to the hypoblast; the 

 conjoined meso-epiblastic layer is the somatopleure; the 

 meso-hypoblastic the splanchnopleure. The proximal parts 

 of the somatopleure (i.e., the regions nearest the central 



