THE REGION OF THE SPINAL CORD 401 



dips into the appendage, and gives origin to the muscles of the 

 appendage. In the vertebrate, after the somatic dorsal part or 

 myoccele has separated off, a ventral part is left, which forms a 

 nephroccele in the trunk-region, and gives origin to the splanchnic 

 striated muscles in the cranial region, i.e. to the muscles which, 

 according to my theory, were once appendicular muscles. This 

 ventral nephroccelic part is divisible in the trunk into a segmented 

 part, which forms the excretory organs proper, and an unsegmented 

 part, the metaccele or true body-cavity of the vertebrate. 



This comparison of the procoelom of the vertebrate and arthropod 

 signifies that the vertebrate metaccele was directly derived by ventral 

 downgrowth from the arthropod nephroccele, so that if, as I suppose, 

 the vertebrate nervous system represents the conjoined nervous 

 system and alimentary canal of the arthropod, then the vertebrate 

 metaccele, or body-cavity, must have been originally confined to the 

 region on each side of the central nervous system, and from this 

 position have spread ventrally, to enclose ultimately the new-formed 

 vertebrate gut. This means that the body-cavity (metaccele) of the 

 vertebrate is not the same as the body-cavity of the annelid, but 

 corresponds to a ventral extension of the nephroccele, or ventral part 

 of such body-cavity. 



Such a phylogenetic history is most probable, because it explains 

 most naturally and simply the facts of the development of the verte- 

 brate body-cavity ; for the mesoblast always originates in the neigh- 

 bourhood of the notochord and central nervous system, and the lumen 

 of the body- cavity always appears first in that region, and then 

 extends laterally and ventrally on each side until it reaches the most 

 ventral surface of the embryo, thus forming a ventral mesentery, 

 which ultimately disappears, and the body-cavity surrounds the gut, 

 except for the dorsal mesentery. Thus Shipley, in his description of 

 the formation of the mesoblastic plates which line the body- cavity in 

 Ammoccetes, describes them as commencing in two bands of meso- 

 blast situated on each side, close against the commencing nervous 

 system: — 



" These two bands are separated dorsally by the juxtaposition of 

 the dorsal wall of the mesenteron and the epiblast, and ventrally by 

 the hypoblastic yolk- cells which are in contact with the epiblast 

 over two-thirds of the embryo. Subsequently, but at a much later 

 date, the mesoblast is completed ventrally by the downgrowth on 



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