ORGANIC EVOLUTION 



199 



FiG. 124. A young salamander larva, show- 

 ing gill slits (5). b. blastopore; /, 2, j, fore-, 

 mid- and hind-brain, respectively. 



closure together of two folds about a neural groove, and that 

 the tube thus formed then sinks into the body wall. The 



diagrams of figure 122 

 show how it is over- 

 grown, first by the 

 ectoderm and later by 

 mesoderm, and re- 

 moved from the surface. 

 The original neural 

 groove thus closed be- 

 comes the central canal 

 of the spinal cord. It is 

 lined with a little bit of the epidermal layer of the ectoderm, 

 carried in from the surface. A small ridge of nerve cells 

 that arises each side of the tube dorsally (fig. 122c) 

 becomes divided with growth into pieces corresponding in 

 pairs to the body segments; and when later, nerves grow 

 out as processes from the cells, these pieces become located 

 upon the dorsal roots of the spinal nerves and become the 

 ganglia (fig. 117) hitherto noticed. 



Quite early in . its development the axis becomes feebly 

 marked off into three successive tracts, which correspond to 

 the fore brain, mid brain and hind brain of the adult sala- 

 mander (figs. 124 and 125). The fore brain becomes bilobed 

 by a dilatation on either side of the median plane, and by 



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:*w 



'3. 





^^ 



Fig. 125. Older salamander larva, showing gills, /. 2. j, fore-, mid- and hind- 

 brain. 



^ 



