68 



HISTORY OF THE HUMAN BODY 



In it the region of the epimeres becomes easily distinguished 

 by a great increase in the thickness of the layer, and an indi- 

 cation of the separate diverticula appears through a series of 

 transverse fissures, which divide the mass into separate square 

 blocks, the so-called mesodermic somites. These first appear 

 at about the middle of the body, and are added to progres- 



FIG. 16. Three early vertebrate embryos, showing mesodermic somites. 



(a) turtle [after MITSUKURI]. (&) chick [after DUVAL]. (c) pig [after KEIBEL]. 

 nt, nerve cord (brain) ; nt', nerve cord (spinal cord) ; ms, mesodermic somites; e, ear; 

 yv, yolk veins. 



sively both anteriorly and posteriorly until the full number is 

 reached. The meso-hypomeric region remains for a time as a 

 single undivided layer, but ultimately splits into two, outer and 

 inner, containing between them a single undivided space, the 

 future ccelom. This latter is here called a schizoccele, in re- 

 spect to its mode of origin. 



There is thus attained in the higher vertebrates a much 

 shortened and greatly modified method of producing the ele- 



