228 Royal Society : — 



lies between the origin of the nerve h and the eye, answers to all 

 that part of tlie brain which lies between the origin of the seventh 

 nerve of Pttromi/zon and the optic nerve. Conseipiently the lateral 

 walls of the neural canal in the same region answer to that region 

 of the skull in Rtromi/zon which lies between the origin of the 

 seventh and the origin of the optic nerve. Hence, as each myo- 

 tome of Amjihio.vHs represents the corresponding portion of a 

 protovertebra, it follows that the same region of the skull in the 

 Lamprey and other Vertebrata represents, at fewest, six protover- 

 tebra', almost all traces of which are lost, even in the embryo con- 

 dition of the higher Vertebrata. 



It may further be concluded that the several pairs of nerves 

 which leave the cerebro-spinal axis, between those which answer 

 to the port'to dura and the optic nerve, in Amjihioxas, are repre- 

 sented by the third, foiu-th, fifth, and sixth pairs of cranial nerves 

 of the higher Vertebrata. The nerve a, in fact, has the charac- 

 teristic course and distribution of the orbito-nasal division of the 

 trigeminal ; while, without at present dravnng a closer parallel, 

 it is easy to see that the nerves b,c,d,e,f, and g, with their 

 respective myotomes, supply the requisite materials for meta- 

 morphosis into the oculomotor, pathetic, trigeminal, and abduceus 

 nerves, with the muscles of the eye and of the jaws, in the more 

 differentiated vertebrate types. 



Thus that part of the cerebro-spinal axis of Am2)hioxus which 

 lies in front of the seventh myotome answers to the praeauditory 

 part of the brain in the higher Vertebrata, and the corresponding 

 part of the head to the trabecular region of the skull in them. 

 On the other baud, from the seventh myotome backwards, a certain 

 number of segments answer to the postauditory, or parachordal, 

 region of the skull of the higher Vertebrata. 



The answ er to the question, how many ? involves sundry con- 

 siderations. It must be recollected that though the branchial 

 chamber of Ampldoxus is the homologue of the branchial chamber 

 of other Vertebrata, it does not necessarily follow that the im- 

 perfect branchial skeleton of Amplnoxus corresponds with their 

 branchLal skeleton. The branchial skeleton of the higher A^erte- 

 brata consists of cartilaginous rods, which seem to be developed 

 in the somatopleure, and to be homologous \\ ith the ribs, while the 

 branchial skeleton of Amiihio.vKs consists of fibrous bauds appa- 

 rently developed in the splanchnopleure. 



The branchial arches of the higher Vertebrata, in accordance 

 \«ith their essentially costal nature, receive their innervation 

 from the glosso-pharyngeal and pueumogastric nerves, which are 

 homologues of spinal nerves ; and, in seeking for the posterior 

 limits of that region in Amphioxus which corresponds with the 

 skull and brain in other Vertebrates, we must only take into 

 account as many pairs of those nerves which arise from the 

 cerebro-spinal axis as we know are, in the A^ertebrata next above 

 Amphioxus, devoted to the branchial arches. In none of these 

 are there more than seven pairs of branchial arches ; so that not 



