244 HOWARD AYERS. [Vol. IV. 



with the primitive organs, persisting with no change of function 

 from those which have appeared much later and in a modified 

 condition of the body. The brain is of the former class, the 

 cranium of the latter. As an illustration, we may take the 

 cases found in nature of a completely segmented vertebral 

 column and its separation from the skull by an articulation, 

 and the opposite condition of a continuous cartilaginous skele- 

 ton, including vertebral column and cranium. The explanation 

 current is, that in the latter case we have to do with secondary 

 fusion, and that we have here a modified condition derived from 

 the typically segmented condition by a lack of development 

 of the articulation. I consider it the true view, that a continu- 

 ous cartilaginous skeleton has been, and is in some living cases, 

 the primitive and normal condition, though of course I do not 

 deny the production of more or less continuous portions of the 

 skeleton by a process of secondary fusion. 



The segmentation of the contained or enveloping organs does 

 not predicate the segmentation of the containing or enveloping 

 structure, any more than the formation of an ear capsule of a 

 mammal, out of separate bones, predicates the segmentation of 

 the sense organ contained. 



Q. The head cavities, or spaces included within the mesoblastic 

 sofnites occnrring in the head region, possess relatively the great- 

 est importance in an acraniate stage before a skull or anything 

 comparable with a primordial craniiifn has made its appearance. 

 This is true from the ontogenetic as well as phylogenetic stand- 

 point. 



R. The hypophysis is a structure which arose in the vertebrate 

 phylum long after the chorda was established, as Amphioxus 

 proves, and was connected ijt an important way with the infundib- 

 ulum. It arose as an organ of taste, and the infundibulum, was 

 its nerve. 



S. The optic chiasm, {the trochlear chiasm as well) has arisen 

 within the vertebrate group above the Amphioxus condition and in 

 the following manner : — 



The fibres supplying the pigment spots (or muscle) arose from 

 ganglion cells of the multipolar kind, and as we know in Amphi- 

 oxus and Cyclostomes, these fibres cross the middle line not 



