WORMS TO VERTEBRATES 75 



marrow running down the back. The brain of all ex- 

 cept the very lowest vertebrates consists of four por- 

 tions : 1. The cerebrum, or cerebral lobes, or simply 

 "forebrain," the seat of consciousness, thought, and 

 will, and from which no nerves proceed. Whether the 

 primitive vertebrate had any cerebrum is still uncer- 

 tain. 2. The mid-brain, which sends nerves to the 

 eyes, and in this respect reminds us of the brain of in- 

 sects. Its anterior portion appears from embryology to 

 be very primitive. 3. The small brain, or cerebellum, 

 which in all higher forms is the centre for co-ordina- 

 tion of the motions of the body. 4. The medulla, 

 which controls especially the internal organs. The 

 spinal marrow, or that portion of the nervous system 

 which lies outside of the head, is at the same time a 

 great nerve-trunk and a centre for reflex action of the 

 muscles of the body. But the development of these 

 distinct portions and the division of labor between 

 them must have been a long and gradual process. 



We have every reason to believe that here, as in in- 

 sects, the head has been formed by annexation of seg- 

 ments from the rump and the fusion of their nervous 

 matter with that of the brain. But here, instead of 

 only three segments, from nine to fourteen have been 

 fused in the head to furnish the material for the brain. 

 Notochord and backbone may be the most striking 

 and apparent characteristic of vertebrates, but their 

 predominant characteristic is brain. On this system 

 they lavished material, giving it from three to four 

 times as much as any lower or earlier group had done. 

 They very early set apart the cerebral lobes to be the 

 commander-in-chief and centre of control for all other 

 nerve-centres. To this all report, and from it all di- 



