118 ZOOLOGY SECT. 



system and notochord, and a ventral portion, the lateral plate, 

 bounding the mesenteron. The vertebral plate undergoes meta- 

 meric segmentation, becoming divided into a row of squarish 

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

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

 ectoderm, a splanchnic (spl), to the endoderm. The space between 

 the two is the coelome (coel), which is thus a schizocoele, or cavity 

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

 in the Lampreys (p. 132), at no stage in communication with 

 the mesenteron, like the coelomic pouches of Amphioxus. The 

 dorsal portion of the coelome 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 protovertebrse the myomeres are formed, from 

 their ventral portions the vertebrae. 



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 

 extending 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. 



