INTERNAL ANATOMY. 91 



the ventral tube, so as to form a vesicular dilatation cov- 

 ered over by a thin membrane. The region of the nerve- 

 tube, over which this dorsal dilatation extends, has been 

 compared by HATSCHEK, who discovered it, to the medulla 

 oblongata of the craniate Vertebrates, which is similarly 

 roofed in only by membrane. In the fully grown condi- 

 tion, however, it seems to be largely obliterated by the 



Fig. 46. Transverse section through the spinal cord between the second and 

 third sensory roots. (After ROHDE.) 



g.c. Dorsal aggregation of ganglion-cells (extending between the second and 

 fifth pairs of sensory nerves ; a somewhat similar group of ganglion-cells occurs on 

 ventral side of nerve-cord below the central canal between the fourth and sixth 

 sensory nerves.) 



d.r. Dorsal root. s.f. Supporting fibres, c.c. central canal ; in this case equally 

 wide throughout its entire height, and so all along the spinal cord. s/i. Sheath of 

 nerve-cord. 



development of a mass of large ganglion-cells which ex^ 

 tend backwards as far as the fifth pair of sensory nerves 

 (Fig. 46). 



All there is of a brain in Amphioxus is shown in Fig. 

 45. The cerebral vesicle is a plain cavity without any 

 true subdivision into ventricles. 9 In the development of 



