122 ZOOLOGY 



SECT. 



bounding the mesenteron. The vertebral plate undergoes meta- 

 meric segmentation, becoming divided into a row of squarish 

 masses, the protovertebrce or mesodcrmal segments (B, pr. v.) : the 

 lateral plate splits into two layers, a somatic (som.}, adherent to the 

 ectoderm, a splanchnic (&pl.) t to the endoderm. The space between 

 the two is the ccelome (cceL), which is thus a schizocwle, or cavity 

 hollowed out of the rnesoderm, and is, except in the head-region 

 in the Lampreys (p. 136), at no stage in communication with 

 the mesentercn, like the ccelomic pouches of Amphioxus. 

 The dorsal portion of the ccelome assumes the character of a 

 series of paired diverticula of the main ventral part, each situated 

 in the interior of a protovertebra ; but such an arrangement is 

 temporary, and these protovertebral cavities early disappear. 

 From the dorsal portions of the protovertebrae the myomeres are 

 formed, from their ventral portions the vertebra. 



The development of the principal organs has been described, in 

 general terms, in the preceding account of the organs themselves : 

 it will be convenient to defer further consideration of this subject 

 until we come to deal with the development of the various types 

 of Craniata, and with the embryological characteristics of the 

 classes and sub-classes. 



Metamerism. A tendency, more or less strongly marked, to 

 a serial repetition of parts is to be observed in a number of 

 different systems of organs. Instances of this have already been 

 pointed out in the skeleton, and the muscular, nervous and excretory 

 systems. This phenomenon seems to lead to the conclusion that 

 the structure of the Craniata can be understood only when they 

 are regarded as metamerically-segmented animals. The phase 

 of metamerism presented by the Craniata is, however, widely 

 different from that which prevails in the segmented Invertebrates. 

 In the latter the segmentation is usually quite distinctly pro- 

 nounced externally, and it may involve a metameric division ex- 

 tending to the ccelome as well as to the various systems of internal 

 organs. In the Craniata, on the other hand, segmentation is 

 never visible on the exterior, and in the adult condition the 

 ccelome never shares in the division. Even in the case of the 

 organs which present metameric characters, the metamerism often 

 appears indefinite and uncertain : thus, as already pointed out, 

 the segmentation of the spinal column, which in the adult is the 

 most pronounced of all, does not coincide with the segmentation 

 of the muscular and nervous systems. Yet when we take the 

 phenomena of embryonic development into account, it becomes 

 sufficiently clear that in the Craniata we have to do with animals 

 possessing a metameric segmentation of the same general type as 

 that possessed by Amphioxus, and that the apparent anomalies 

 are due to processes of secondary modification. 



It is in the trunk region that the metamerism is most strongly 



