408 



CH^iTOPODA. 



each side of a ventral ciliated furrow, first as a single row of epi- 

 blast cells, and subsequently as several rows (fig. 240, Vg). While 

 still united to the external epi- 

 blast, they extend themselves be- 

 low the cells lining the ventral 

 furrow, and unite into a single 

 nervous band, which however 

 exhibits its double origin by its 

 bilobed section. Before the two 

 cords unite, the groove between 

 them becomes somewhat deep, 

 but subsequently shallows out 

 and disappears. The nervous 

 band, before separating from the 

 epiblast, exhibits, in correspond- 



FIG. 240. SECTION THROUGH 

 PART OF THE VENTRAL WALL OF THE 

 TRUNK OF AN EMBRYO OF LUMBRI- 

 cus TRAPEZOIDES. (After Kleinen- 

 berg.) 



;;/. longitudinal muscles ; so. so- 

 matic mesoblast ; sp. splanchnic me- 

 soblast; hy. hypoblast; Vg. ventral 

 nerve-cord ; vv. ventral vessel. 



ence with the mesoblastic segments, alternate swellings and 

 constrictions. The former become the ganglia, and the latter the 

 connecting trunks. 



As soon as the cord becomes free from the epiblast, it 

 becomes surrounded by a sheath, formed of somatic mesoblast. 

 In each of the ganglionic enlargements there next appears on 

 the dorsal surface a pair of areas of punctiform material, the 

 substance of which soon differentiates itself into nerve-fibres. 

 These areas, by uniting from side to side, give rise to the 

 transverse commissures, and also by a linear coalescence to the 

 longitudinal commissures of the cord. The cellular parts of the 

 band surrounding them become converted into a ganglionic 

 covering of the cord. 



In each ganglion the cells of this ganglionic investment 

 penetrate as a median septum into the cord. A fissure is next 

 formed, dividing this septum into two ; it is subsequently 

 continued for the whole length of the cord. 



Arthropoda. In the Tracheata and the Crustacea the 

 development of the ventral cord is in the main similar to that in 

 the Chaetopods, while that of the supracesophageal ganglia is as 

 a rule somewhat more complicated. No such clear evidence of 

 an independent development of these two parts, as in the case 

 of the Chaetopods, has as yet been produced. 



The most primitive type of nervous system amongst the 



