THE REGION OF THE SPINAL CORD AIlI 
branchie, the pronephric tubules, and the sense-organs of the lateral 
line system. 
Such an explanation signifies that the somatic trunk-musculature 
of the vertebrate was derived from the dorsal longitudinal muscula- 
ture of the body of the arthropod, and not from the ventral longitu- 
dinal musculature, and that therefore in the primitive arthropod stage 
the equivalent of the myotome of the vertebrate did not give origin 
to the ventral longitudinal muscles of the invertebrate ancestor. 
Now, as I have said, von Kennel states that in the procelom 
of Peripatus a dorsal part (III. in Fig. 157) is cut off which gives 
origin to the dorsal body-musculature, while the ventral part which 
remains (I, and II. in Fig. 157) gives origin in its appendicular 
portion (I.) to the muscles of the appendage, and presumably in its 
ventral somatic portion (II.) to the ventral longitudinal muscles of 
the body. This dorsal cut-off part might be called the myotome, in 
the same sense as the corresponding part of the proccelom in the 
vertebrate is called the myotome. In both cases the muscles derived 
from it form only a part of the voluntary musculature of the animal, 
and in both cases the muscles in question are the dorsal longitudinal 
muscles of the body, to which must be added the dorso-ventral body- 
muscles. Now, the whole of my theory of the origin of vertebrates 
arose from the investigation of the structure of the cranial nerves, 
which led to the conception that their grouping is not, like the 
spinal, a dual grouping of motor and sensory elements, but a dual 
grouping to supply two sets of segments, characterized especially by 
the different embryological origin of their musculature. The one set 
T called the somatic segmentation, because the muscles belonging to 
it were the great longitudinal body-muscles; the other I called the 
splanchnic segmentation, because its muscles were those connected 
with the branchial and visceral arches. According to my theory, 
this latter segmentation was due to the segmentation of the appen- 
dages in the invertebrate ancestor; and in previous chapters, dealing 
as they do with the cranial region, attention was especially directed 
to the way in which the position of the striated splanchnic muscula- 
ture could be explained by a transformation of the prosomatic and 
mesosomatic appendages. Now, I am dealing with the metasomatic 
region, in which it is true the appendages take a very subordinate 
place, but still something corresponding to the splanchnic segments 
of the cranial region might fairly be expected to exist, and I therefore 
