358 Mr. H. G. Seeley on a Theory 



be maintained that the inferior arch of a cervical vertebra of a bird 

 differs less from the inferior arch of a dorsal vertebra than does 

 the ordinary upper arch of a vertebra from the upper arch of a 

 segment of the skull. In the thoracic region the growth and 

 development of viscera is chiefly in depth, as is the weight of the 

 lungs ; and in Amphioocus lanceolatus the notochord extends 

 anterior to the neural cord, whereas in mammals, even in a very 

 early embryonic state, the neural rudiment which becomes the 

 brain is prolonged far in front of the notochord; and thus it is 

 seen that with its development in height the brain undergoes a 

 development in length, which the thorax did not. And nothing 

 can be more evident than that, restrained by the structures in 

 front and by the vertebrae behind, the growth in length must 

 exercise a pressure and tension in that direction exactly corre- 

 sponding to the forces which gave rise to the epiphysial bones 

 which roof in the brain as it developes in height. And there- 

 fore, since by the influence of such enormous and equable 

 pressure and tension epiphyses are developed in height, exactly 

 the same forces exerted in length cannot but have produced 

 epiphyses at each end ; and so, remembering how, up to a 

 certain point, the plan of the brain and the spinal cord must 

 have been the same, it is curious to observe that while the basi- 

 sphenoid developes the basioccipital and prcsphenoid for its 

 epiphyses much after the plan of an ordinary centrum, the bones 

 of the neural arch also develope epiphyses in length just as they 

 do in height, as we saw was the case with some fishes — the en- 

 tire occipital segment answering to the posterior epiphysis, and 

 the entire frontal segment being the anterior epiphysis of the 

 parietal segment of the skull. And accordingly it is found that 

 the elementary bones of these epiphyses converge and close in 

 the brain at both ends, thus demonstrating that they owe their 

 growth to its growth, and extend no further than they are forced 

 by its pressure ; and therefore, though the skull will obviously 

 develope quite regardless of the degree of growth in the several 

 parts of the brain, by the simple law of inheritance, yet in many 

 cases the relative size of several bones will be found to vary with 

 the size of the division of the brain which is underneath them. 

 Thus Mr. Robertson remarks that fishes may be divided into a 

 sluggish gi'oup, typified by Lophius, in which the cerebellum is 

 small, and an active group, in which the cerebellum is large, 

 typified by the Tunny ; and finds that in skulls of equal length, 

 the occipital segment of the skull measures 4^ inches long in the 

 Tunny, while in Lophius it only measures 2 inches : and, ascend- 

 ing in organization, it is seen that as the brain rapidly expands, 

 bones which before, in the lower forms, were quite exterior to 

 the skull become gradually introduced to form part of the 

 cranial walls. 



