884 EEPRODUCTION, DEVELOPMENT, GROWTH AND DEATH [CH. LIX. 



protovertebral somites is the intermediate cell mass. Soon after its 

 formation the lateral mesoblast is cleft into two layers, and the space 

 which appears between the two layers is called the coelom (figs. 568, 

 569). The outer or somatic layer of the mesoblast adheres to the 

 epiblast; the two together form the somatopleur. The inner or 

 splanchnic layer fuses with the hypoblast to form the splanchnopleur. 



FIG. 565. Diagram of a surface 

 view of a young mammalian 

 blastula. 1, Germinal area. 

 A, line of section represented 

 in fig. 567. 



Flo. 567. 7 Diagram of a 

 section of the mammalian 

 blastula shown in fig. 566 

 along the line A. 1, Ger- 

 minal area ; 2, epiblast ; 

 3, inner cell mass. 



Whilst the mesoblast is extending and cleaving, the neural ridges, 

 which are longitudinal epiblastic upgrowths on each side of the 

 middle line, gradually grow in height, and their free margins turn 

 inwards and fuse together. This fusion commences in the cervical 

 region, and extends forwards and backwards, and when it is com- 

 pleted the neural groove is converted into a closed tube, the neural 



6 



FIG. 568. Diagram of a transverse section 

 through a mammalian blastoderm after the 

 formation of the three primary layers. 1, 

 Primitive groove ; 2, primitive streak ; 3, 

 epiblast; 4, mesoblast; 5, hypoblast; 6, 

 coelom ; 7, archenteron. 



7 B 



FIG. 569. Diagram of a transverse section 

 through a mammalian blastoderm, show- 

 ing the neural ridges. 1, Neural groove ; 

 2, neural ridge ; 8, epiblast ; 4, somatic 

 mesoblast ; 5, splanchnic mesoblast ; 6, 

 hypoblast ; 7, somatopleur ; 8, splanch- 

 nopleur; 9, notochord ; 10, ccelom; 11, 

 archenteron. 



canal, which is the central canal of the nervous system. In the 

 embryo at this period there are, therefore, three cavities : (1) The 

 neural canal lined by epiblast ; (2) The coelom or space in the meso- 

 blast; (3) The archenteron within the hypoblast. While these 

 changes are in progress the embryonic area begins to fold off from 

 the rest of the embryo. A sulcus appears all round the margins 

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



