xii DEVELOPMENT 203 



will be seen (Fig. 65) that the medullary groove is lined 

 by ectoderm and that therefore the whole central nervous 

 system is ectodermic in origin ; and this is also true of the 

 nerves. 



A longitudinal thickening of the endoderm on the dorsal 

 side of the archenteron becomes constricted off as a solid 

 rod of cells, lying between the medullary cord and the 

 archenteron. This is the notochord (Figs. 64, K, and 65, 

 ncti) ; it forms the primary axial skeleton around which the 

 vertebral column is subsequently developed. The meso- 

 derm is for some time a solid mass occupying all the space 

 between the ectoderm and endoderm, so that there is no 

 body-cavity. But at a later stage a cavity appears divid- 

 ing the mesoderm into two layers, one in contact with the 

 ectoderm (parietal layer, Fig 65, mes l \ the other with the 

 endoderm (visceral layer, mes 2 ). The space between them 

 is the ccelome (c). The dorsal part of the mesoderm on 

 either side of the medullary cord and notochord becomes 

 divided transversely, and gives rise to muscle-segments or 

 myomeres (Fig. 64, L). 



The embryo now begins to elongate in a definite direc- 

 tion (Fig. 64, j) : its dorsal surface, still marked by the 

 medullary groove, which now soon becomes closed, is slightly 

 concave, its ventral surface very convex. The blastopore 

 closes up, the yolk-plug becoming entirely covered by ecto- 

 derm ; but for a short time the medullary canal and 

 archenteron still communicate by the neurenteric canal 

 (K, n.e.c.}. This soon disappears, and at the hinder end 

 of the embryo a conical outgrowth forms the rudiment 

 of the tail (). The opposite end is rounded, and on its 

 ventral surface appears a little half-moon shaped groove 

 (which afterwards becomes subdivided into two, Fig. 66, 

 c), the rudiment of the sucker by which, the tadpole 



