CH^TOPODA. 



339 



is the nervous system ; the origin of which from this layer was 

 first established by Kowalevsky (No. 342). 



It arises 1 (Kleinenberg, No. 341) from two at first quite 

 distinct structures, viz. (i) the supra-cesophageal rudiment 

 and (2) the rudiment of the ventral cord. The former of 

 these takes its origin as an unpaired dorsal thickening of 

 the epiblast at the front end of the head (fig. 156, e.g.), which 

 sends two prolongations downwards and backwards to meet the 

 ventral cord. The latter arises as two independent thickenings 

 of the epiblast, one on each side 

 of the ventral furrow (fig. 157, Vg}. 

 These soon unite underneath the 

 furrow, in the median line, and 

 after being differentiated into seg- 

 mentally arranged ganglionic and 

 interganglionic regions become 

 separated from the epiblast. Both 

 the supra-cesophageal and ven- 

 tral cord become surrounded by a 

 layer of somatic mesoblast. The 

 junction between the two parts of 

 the central nervous system takes 

 place comparatively late. 



The mesoblast. It is to Kowalevsky (No. 342) and Klein- 

 enberg (No. 341) that we mainly owe our knowledge of the history 

 of the mesoblast. The fundamental processes which take place are 

 (i) the splitting of the mesoblast into splanchnic and somatic 

 layers with the body cavity between them, (2) the transverse 

 division of the mesoblast of the trunk into distinct somites. 



The former process commences in the cephalic mesoblastic 

 commissure, where it results in the formation of a pair of cavities 

 each with a thin somatic and thick splanchnic layer (fig. 156, 

 cc) ; and thence extends gradually backwards into the trunk 

 (fig. 141 C, //). In the trunk however the division into somites 

 precedes the horizontal splitting of the mesoblast. The former 

 process commences when the mesoblastic bands form widish 

 columns quite separate from each other. These columns become 



FIG. 157. SECTION THROUGH PART 

 OF THE VENTRAL WALL OF THE 

 TRUNK OF AN EMBRYO OF LUMBRI- 

 cus TRAPEZOIDES. (After Kleinen- 

 berg.) 



m. longitudinal muscles ; so. so- 

 matic mesoblast ; sp. splanchnic me- 

 soblast ; hy. hypoblast ; Vg. ventral 

 nerve cord ; vv. ventral vessel. 



1 For further details, vide general chapter on Nervous System. 



22 2 



