242 HOWARD AVERS. [Vol. IV. 



N. The neural axis of all vertebrates is co-extensive with that 

 of the chorda, or vice versa, since the neural axis is phylogenetically 

 as well as ontogenetically the older structure. 



In this I can confirm Keibel's observation on mammals, by 

 my own on Sharks. The same is true of the early larval Cyclos- 

 tomata and Amphioxus. My own results, the outcome of a 

 study of Acanthias vulgaris and Galeus canis, at the Maine Bio- 

 logical Laboratory, Wood's Holl, Mass., were obtained several 

 months before the publication of Dr. Keibel's paper. This rela- 

 tion of chorda and neural axis exists, not only in all the higher 

 vertebrates, but in Amphioxus as well, during the stage in which 

 the organs in question arrive at their complete separation from 

 the surrounding tissues ; but while in higher vertebrates the 

 notochord suffers a shortening of its anterior end, and not infre- 

 quently of its posterior end also, in Amphioxus the anterior 

 end secondarily grows out into a process for the support of the 

 pointed anterior end of the head. 



A cephalic flexure occurs in both, and is more or less com- 

 pletely obliterated in both in the later stages, though from dif- 

 ferent causes. 



In no craniate vertebrate does the notochord extend the 

 entire length of the embryo when first formed, but ends in 

 an undifferentiated mass of cells placed below and in front 

 of the neural axis, and later grows out in front of it. This out- 

 growth is soon overcome by the much greater and more rapid 

 growth of the brain. The conditions in Amphioxus are simpler 

 in that the notochord is, from the first, distinct quite to the end 

 of the neural axis, the anterior termination of the body being 

 relatively much larger than in embryo of the higher vertebrata 

 of corresponding stages. 



O. The pituitary prominence of the skulls of vertebrates does 

 not mark a fixed point, as the relation of the anterior end of the 

 chorda and of the hypophysial organs clearly proves. 



I reserve a detailed consideration of this proposition for my 

 illustrated publication. 



P. /;/ the discussion of the segmentation of the head it has 

 become necessary to deny any segmejttal value whatsoever to any 

 portion of the chondro, — or ossicranium. They have no greater 

 segtnental value than the intestine. And all apparent segmental 



