IV. VERTEBRATA 407 



The action of the muscles prevents the formation of a cartilaginous or 

 bony verteljral continuum such as the notochord and skeletogenous 

 layer are. It produces at intervals joints or flexible parts separating the 

 cartilaginous or bony column into vertebnc. Naturally these ilexible 

 portions cannot coincide with the boundaries of the muscles, but must 

 lie between them; in other words, muscle segments and skeletal segments 

 — myotomes and sclerotomes — must alternate. Segmentation is lacking in 

 the cranium, since the myotomes here have no locomotor significance, 

 are reduced, and only small remnants of them persist. 



In the mammals only a little of this segmental arrangement of muscles 

 is recognizaljle, a result of the development of the appendages; and the 

 more these gain in importance as the locomotor structures, the more the 

 muscles are modilied and grouped for the service of the limbs, so that only 

 the intercostals and a part of the muscular system to the sides of the 

 vertebral column show clearly the primitive metamerism. Yet in all 

 vertebrate embryos the muscles appear at first strictly segmental, in the 

 form of the primitive somites (fig. 524), formerly called protovertcbra. 

 Parts, like buds, separate from the ventral sides of the protovertebne 

 and furnish the musculature of the limbs. 



Another important point in the musculature lies in the fact that it is dorsal in 

 origin and therefore in fishes is largely dorsal in position throughout life. The 

 muscles which are ventral have largely been transferred from the back, and the 

 cause of the migration is to be recognized to a large extent in the progressive 

 development of the appendages. The dorsal position of the muscles is only a 

 part of a general fact, that the skeletal axis divides the body into a dorsal zone, 

 containing only animal organs, and a ventral zone, chiefly vegetal in character, 

 Besides the muscles, the central nervous system, and the most important sense 

 organs — eyes, nose, ears — belong to the dorsal zone. 



The central nervous system of vertebrates consists of brain and 

 spinal cord. Like that of all chordates it is distinguished from that of 

 other segmented animals-^annelids, arthropods, in which there is a dorsal 

 brain and a ventral nerve chain — in its purely dorsal position. It is 

 further distinguished from that of all non-chordates by its tubular character, 

 that is, by the presence of a central canal in the axis of cord and brain (fig. 

 79), lined by a special epithelium, the ependyma, and containing a fluid, the 

 liquor cerebrospinalis. This central canal is the result of the mode of devel- 

 opment, the nervous system arising by an inroUing of the ectoderm (fig. 9). 

 A median longitudinal medullary groove arises early in the dorsal ectoderm 

 of the embryo. The floor of this groove, the medullary plate, gradually 

 rolls into a tube, the edges curving upwards and meeting above. In this 

 process there is developed, as in the tunicates (fig. 500), a neurenteric 

 canal connecting the hinder end of the neural canal with the digestive 



