36 PROCEKUINGS OI Till 



these structures liuve not bneii inheritotl Iroiii any Invertebrate 

 ancestor at all, but have arisen cjuite independently within the 

 A'ertebrate group. 



In connection with Dr. Gaskell's theory, the question is some- 

 times asked: — If the cavity of the central nervous system of the 

 Vertebrate, with its lining epithelium, has not been derived from 

 the alimentary canal of an Arthropod ancestor, how do you 

 account for its existence, and how do you account for the existence 

 of the choroid plexuses ? To the zoologist, of course, this 

 question presents no difficulty. One of the commonest ])heno- 

 mena of development throughout the Animal Kingdom is the 

 incrense of surface by the forniation of folds. We are familiar 

 with it in glandular tissues and in respiratory tissues, and we are 

 familiar with it also in the formation of the central nervous 

 system of various Invertebrates, as Professor MacBride has 

 already pointed out. jN'o one doubts, moreover, that this is the 

 explanation of the convolutions of the brain in higher A^ertebrates. 

 Why then object to apply the same principle in expltmation of 

 the origin of the Vertebrate nervous system by invagination of the 

 superficial epiblast ? The Vertebrates inherited from their In- 

 vertebrate, worm-like ancestors, this characteristic mode of 

 forming the central nervous system, which naturally resulted in 

 the development of a hollow^ tube ■with at first a narrow lumen. 

 Eurther evolution of the nervous system was brought about 

 primarily by the increase in number of the nerve-cells and the 

 consequent thickening of the wall of the neural tube. Ic will, of 

 course, be asked by the supporters of Dr. Gaskell's theory, why 

 has the cavity of the original neural tube increased to such 

 enormous dimensions in the case of the ventricles of the brain ? 

 Here again I do not see any difficulty. The great mass of nerve 

 tissue formed in the brain requires some very well developed 

 system for nutrition and respiration. This is primarily effected 

 of course by the cerebral blood-vessels ; but we have also the 

 cerebro-spinal fluid, with whieh the ventricles of the brain and the 

 canalis centralis of the spinal cord are filled, and which probably 

 exercises an important respiratory and possibly also other 

 functions. I suppose Dr. Gaskell will hardly ask us to look upon 

 the cerebro-spinal fluid as representing the digestive juices which 

 were poured into the stomach of the ancestral Arthropod. 



What about the choroid plexuses, then ? Here, again, we have 

 a beautiful illustration of the principle of folding in order to 

 increase surface, a folding which is quite inexplicable except on 

 the assumption that the choroid j)lexuses fulfil some very im- 

 portant function in connection with the cerebro-spinal fluid into 

 which they dip. They are, as everyone knows, extraordinarily 

 vascular (which the wall of the Arthropod stomach is not), and 

 they probably constitute a kind of intra-cerebral gills concerned in 

 the respiration of the cerebro-spinal fluid ; they may also have 

 other functions in connection with this important fluid. 



