258 THE EVOLUTION OF MAN. 



The dorsal half encloses the spinal cavity, which lies above 

 the notochord, and contains the tube-shaped central nerve 

 system, the spinal marrow or spinal tube. The ventral 

 half, on the other hand, encloses the much larger intestinal 

 or ventral cavity, which lies below the notochord, and con- 

 tains the intestinal canal with all its appendages. 



The spinal, or medullary tube, as the central nerve 

 system or mental organ of Vertebrates is called in its primi- 

 tive condition, consists in Man, as in all higher Vertebrates, 

 of two very different parts : the large brain lying within the 

 skull, and the long spinal cord which extends from the brain 

 along the whole back (Plate V. Fig. 16, r)i). But no part of 

 this structure is seen in our primitive vertebral type. In this 

 the highly important mental organ, which occasions the feel- 

 ing, willing, and thinking of the Vertebrate, appears in an 

 extremely simple form. It is composed of a long cylindrical 

 tube which passes lengthwise through the body immediately 

 above the notochord, and encloses a narrow central canal filled 

 with fluid (Fig. 52-57, m7'). We find that the Amphioxus 

 at the present day retains throughout life this simplest 

 form of the spinal canal, just as it existed in aU the older 

 and lower Vertebrates (Plate XI. Fig. 15, m). It is enclosed 

 in a tube of skin which proceeds from the immediate 

 surrounding of the notochord, the so-called notochord 

 sheath, and in which, at a later period, the bony vertebrae 

 of the higher Vertebrates are developed. 



Of organs of sense, the parent-form of Vertebrates 

 probably possessed an olfactory groove, as the simplest 

 rudiment of a nose (Fig. 52, 53, na), a pair of eyes (an), 

 and a pair of auditory vesicles {g) of the most simple cha- 

 racter.^^ Some of these organs of sense are not represented 



