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 
myoccle 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, 7.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 proccelom 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 nephroceele, 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 
2D 
