306 EMBBYOLOGY OF THE LOWEE VEKTEBEATES oh. 



of this group of Vertebrates. In tlie Fishes the sternum has not 

 yet made its appearance. 



Skull. — The skull is a mass of condensed and strengthened 

 mesenchyme serving essentially to support and protect the organs 

 of the head. It protects the brain and sense organs : and it forms 

 a support and framework for the masticatory and other apparatus 

 connected with the mouth and pharynx. In correlation with this 

 its characteristics in detail are secondary to characters of the brain 

 and other organs. 



The skeletonization of the mesenchyme does not take place 

 continuously but commences in irregular patches which gradually 

 spread and eventually join together. Though there is frequently 

 considerable agreement between different Vertebrates in the position 

 of the centres of skeleton formation in the head there are in other 

 cases equally well-marked variations between forms known to be 

 phylogenetically closely related. It is as a rule impossible to say 

 definitely whether or not the first appearance of skeleton at 

 particular points is of phylogenetic significance or is on the other 

 hand related merely to existing arrangements of the adult. 



Under the circumstances all that will be attempted here is a 

 short sketch of the general features of cranial development without 

 entering at all into minute detail. For a full and detailed 

 description reference should be made to the admirable work of 

 Gaupp (1906). 



As has already been indicated there is a marked tendency for the 

 arch - elements to undergo fusion towards the head end, the axial 

 skeleton being necessarily rigid instead of flexible in the brain region. 

 Eventually towards the front end of the series both neural arches 

 and vertebral centra become completely fused together to form part 

 of the skull. 



The skull consists in its simplest form primarily of a chondro- 

 cranium — a trough of cartilage, the cavity of which is occupied by 

 the brain and more or less open on its dorsal side. Somewhere 

 about the middle of the floor of the chondrocranium there exists 

 a recess in which rests the infundibulum of the brain, and the 

 portion of cranial floor lying behind this is distinguished by having 

 the notochord embedded in it — this organ having its anterior limit 

 just behind the tip of the infundibulum. We are thus brought 

 into touch with a deep-seated distinction between the posterior or 

 epichordal (chordal — Kolliker) region of the cranium and the anterior 

 or prechordal (Kolliker). We are probably justified in regarding 

 the epichordal region of the cranium as being morphologically a 

 metamorphosed portion of vertebral column in which the processes 

 of fusion, already indicated as frequently occurring in the anterior 

 region, have attained to their maximum. As will be explained later 

 the process of incorporation of a few vertebrae (the number varying 

 in different groups) into the hinder end of the cranium can still be 

 observed in ontogeny and it is probable that during the contemporary 



