588 MESODERM AND CCELOME CHAP. 



extension of the mesoderm between the ectoderm and 

 endoderm. Thus the coelome is formed not as an 

 enterocoele, but as a schizoccele. 



In Vertebrates each mesoderm-band becomes differ- 

 entiated into a dorsal portion abutting against the 

 medullary cord and notochord known as the vertebral 

 plate which soon loses its ccelomic space, and a ventral 

 portion, the lateral plate, which is divided into parietal 

 and visceral layers by the coelome (Figs. 65, 152, and 

 155). The vertebral plate undergoes metameric seg- 

 mentation, becoming divided into a row of squarish 

 masses, the mesodermal segments or " protovertebrce " 

 (pr. v, m. s, ms. s), from the outer parts of which (muscle- 

 plates, Fig. 164, m. p) the muscular segments or myomeres 

 are formed (p. 203), and from the inner parts the vertebral 

 column, the segmentation of which alternates with that 

 of the myomeres (compare p. 441). 



Outline of the Development of the Chief Organs in the 

 Craniata (compare pp. 201-210). The nervous system, 

 as well as the essential parts of the sensory organs, are, 

 as we have seen, in all cases formed from the ectoderm. 

 The development of the central nervous system takes 

 place in essentially the same manner as in the lancelet 

 (P- 579) and frog (p. 202). 



In the chick the ectoderm in front of the primitive streak 

 early becomes thickened along the median line to form the 

 medullary plate (Fig. 153, E) ; this gradually increases in 

 length at the expense of the primitive streak, from the cells 

 of which the greater part of the embryo is eventually 

 developed. A medullary groove, bounded by medullary 

 folds (Fig. i55,Aand B), is formed along the median line of 

 the medullary plate and passes into the primitive groove 

 posteriorly ; while near their anterior ends the folds meet 

 and unite, so as to constrict off a medullary tube from the 

 outer ectoderm (Figs. 155 and 158, A). This closure of the 

 tube then gradually extends both forwards and backwards, 

 the groove remaining open longest at its posterior end. By 



