It arises[141] (Kleinenberg, No. 341) from two at first quite distinct structures, viz. (1) the supra-oesophageal 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, _c.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 segmentally arranged ganglionic and interganglionic regions become separated from the epiblast. Both the supra-oesophageal and ventral cord become surrounded by a layer of somatic mesoblast. The junction between the two parts of the central nervous system takes place comparatively late. [141] For further details, _vide_ general chapter on Nervous System. The mesoblast. It is to Kowalevsky (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 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, _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 subsequently 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 Polygordius 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 mesenteries. The two cavities in the cephalic commissure unite dorsally, but