xii DEVELOPMENT 203 



fore 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 enteron becomes constricted oft" 

 as a solid rod of cells, lying between the medullary 

 cord and the enteron. This is the notochord 

 (Figs. 64, K, and 65, nch) ; it forms the primary axial 

 skeleton around which the vertebral column is subse- 

 quently developed. The mesoderm 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 dividing the mesoderm into 

 two layers, one in contact with the ectoderm (parietal 

 layer, Fig. 65, mes 1 , sow), the other with the endoderm 

 (visceral layer, mes 2 , spl) . The space between them is the 

 ccelome (c, coel). 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 (Figs. 64, L, and 65). 



The embryo soon 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 ectoderm ; but for a short time the medullary 

 canal and enteron still communicate by the neur enteric 

 canal (K, n.e.c) . This soon disappears, and at the hinder 

 end of the embryo a conical outgrowth forms the rudi- 

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

 its ventral surface appears a little half-moon-shaped 

 groove, the rudiment of the sucker by which the 

 tadpole attaches itself to weeds (J and L, sk) ; this 

 afterwards becomes subdivided into two (Fig. 66, C). 

 Just above the sucker a depression the mouth-pit or 

 stomodceum (L, stdm] makes its appearance, and is the 



