THE REGION OF THE SPINAL CORD 4 1 I 



branchiae, 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 procoelom 

 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 procoelom 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 

 I 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 



