408 W. H. GASKELL. 



liver-duct opening into the pyloric end of the cephalic stomach. 

 Further, as I have mentioned in my paper in 'Brain' (2), 

 it is absolutely necessary to explain why so large a portion of 

 the cavity of the brain-case is occupied with this peculiar tissue 

 in the lowest Vertebrates only. The one explanation which 

 is offered, as far as I know, is no explanation at all, viz. that 

 the fat-tissue fills up the space between brain and cranial walls 

 in order to support and steady a brain which is too small for 

 its case. The natural rejoinder to this is, according to modern 

 views of evolution, why is the brain too small for its 

 case ? and why also is not the spinal cord also supported by 

 the same kind of tissue ? — for here, as well as in the brain, 

 we find the cavity in which the cord lies is very much larger 

 than the cord itself; and yet, apart from a few large cells, the 

 only solid structures which are found in the space around the 

 cord are irregular strands of connective tissue, as seen in 

 fig. 19, PL XXVII, formed, according to Ahlborn, of elastic 

 fibres and " Schleimzellen.^' 



In accordance with evolutionary ideas, the natural answer 

 to this question is that this peculiar tissue, which is different 

 from any other, the cells of which appear degenerated, which 

 contains lines of pigment between its cells, which is found 

 only in the lowest Vertebrates, and is gradually pushed out of 

 existence in the higher classes as the brain increases in size, 

 fills up the space around the brain because it represents some 

 pre-existing organ which was of importance to the animal 

 from which the Vertebrate sprang. Such an organ is clearly 

 the cephalic liver of the Crustacean-like ancestor — a conclusion 

 which is in perfect harmony with not only the rest of the 

 arguments in this paper, but also with those which I hope to 

 bring forward in the series of papers following this one. As 

 to the tissue round the spinal cord, it is natural that the cells 

 of the liver should not be found in any great quantity there, 

 and it is possible that the loose connective tissue which is 

 there found is the remnant of the connective and muscular 

 tissues which originally were situated in the neighbourhood 

 of the intestine, along the length of the body of the Crustacean. 



