64 HISTORY OF THE HUMAN BODY 



plicated system of narrow spaces lying everywhere between 

 the other layers. The protoccele is thus called the primary 

 and the metaccele the secondary body-cavity, and it is this 

 latter, the one lined by the mesoderm and included between its 

 two layers, that forms the permanent body-cavity of verte- 



a 



FIG. 14. Diagrammatic cross sections through vertebrate embryos, based 

 upon the conditions found in selachians. [Modified, after VAN WIJHE.] 



(a) Earlier stage, in which the three parts of the mesodermic diverticula are 

 still continuous. (b) Later stage, in which the epimeres of the mesodermic di- 

 verticula have separated from the meso-hypomeres and form a continuous layer 

 around the body, interrupted only at the mid-dorsal and the mid-ventral lines. 



In all the figures the ectoderm is represented by square cells, the endoderm by 

 crossing diagonal lines, the mesoderm by solid black, the mesenchyme by dots. 

 I, epimere; II, mesomere; III, hypomere; a, aorta; g, gonad; *, intestine; m, myo- 

 tome of epimere; me, metaceele (the definite crelom) ; n, nerve cord; nc, notochord; 

 nph, nephridium; sk, sklerotome, the anlage of the axial skeleton; w, protonephrotic 

 duct (Wolffian duct). 



brates, the so-called dcelom or pleuro-peritoneal cavity. The 

 narrowed spaces of the protoccele become filled with embry- 

 onal connective tissue cells, the tnesenchyme -, which never as- 

 sume the form of a definite layer, and are produced by pro- 

 liferation from the mesoderm, and perhaps from the other 

 two as well. Canals are left here and there which in time 

 are built up into a continuous system of vessels, with walls of 



