CH.ETOPODA. 



281 



Uy 



FIG. 157. SECTION THROUGH PART 

 OF THE VENTRAL WALL OF THE TRUNK 

 OF AN EMBRYO OF LuMBKICUS TRAPE- 

 ZOIDES. (After Kleinenberg. ) 



m. longitudinal muscles; so. so- 

 matic mesoblast ; sp. splanchnic rne- 

 soblast; hy. hypoblast; Vg. ventral 

 nerve cord; vv. ventral vessel. 



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 thicken- 

 ings 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 segmentally 

 arranged ganglionic and intergangli- 

 oiiic regions become separated from 

 the epiblast. Both the supra-oesopha- 

 geal and ventral cord become sur- 

 rounded by a layer of somatic meso- 

 blast. The junction between the two 

 parts of the central nervous system 

 takes place comparatively late. 



The mesoblast. It is to Kowa- 

 levsky (No. 342) and Kleinenberg 

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

 mesoblast. The fundamental processes which take place are (1) 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 com- 

 missure, 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 G,pp). 

 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 broken up successively from before 

 backwards into somewhat cubical bodies, in the centre of which a 

 cavity soon appears. The cavity in each somite is obviously bounded 

 by four walls, (1) an outer, the somatic, which is the thickest; 

 (2) an inner, the splanchnic; and (3, 4) an anterior and posterior. The 

 adjoining anterior and posterior walls of successive somites unite 

 together to form the transverse dissepiments of the adult, which subse- 

 quently become very thin and are perforated in numerous places, thus 

 placing in communication the separate compartments of the body 

 cavity. The somites, though at first confined to a small area on the 

 ventral side, gradually extend so as to meet their fellows above and 

 below and form complete rings (fig. 157) of which the splanchnic 

 layer (sp) attaches itself to the enteric wall and the somatic (so) to 

 the epiblast. In Polygordus and probably also Saccocirrus and other 

 forms the cavities of the somites of the two sides do not coalesce ; and 

 the walls which separate them constitute dorsal and ventral mesen- 

 teries. The two cavities in the cephalic commissure unite dorsally, 

 but ventrally open into the first somite of the trunk. 



