334 



]\Ir. W. K. Parker on the Structure of [Xov. 23, 



Then, at the end, as I am spending mj life not to illustrate the cranial 

 morphology o£ this type or of that, but as digging down to find one 

 common root, I have made an incipient attempt at showing what is 

 common to the whole series of the Vertebrates — of the hmin-hearing 

 Vertebrates, at any rate. 



It is evident that beneath the neural axis, which arises in " epiblast," 

 there is a foundation, laid in " mesoblast," of the whole animal, from its 

 snout to the end of its tail. 



This foundation, or rather root-stock, is double, and each moiety lies 

 right and left of a truly azygous structure, the notocliord — a structure 

 which, according to some, arises in the mesoblast also, but which, 

 according to the latest and best observations (namely, those of J\Ir. 

 Balfour), arises, in the Selachians at least, in the lowest layer, the 

 " hypoblast." 



AYhether the notocliord is mesoblastic or hypoblastic, at present is not 

 of vital moment to the morphology of a vertebrated animal : the im- 

 portant points are that the notochord is universal, and that it always 

 passes some distance into the skull. 



There are several important modifications in the region of the head, as 

 compared with the body generally, that make the problem of cranial 

 morphology an extremely difficult one. 



To mention some, there are : — (1) the swelling of the neural axis into 

 three vesicles ; (2) the flexure of the head upon itself ; (3) the develop- 

 ment of three pairs of sense-capsules, that press upon its sides and 

 mingle with its structures ; (4) the union of a palatal diverticulum with 

 the brain to form the pituitary body, thus arresting the median noto- 

 chord ; and (5) the dpng out of the pleuro- peritoneal space in the region 

 of the throat. 



Thus the modif}"ing causes are manifold in the head of a vertebrated 

 animal, — some of them showing their effects very early in the life of the 

 embryo ; whilst others, that relate to the specializations of the parts of the 

 cranium and of the parts of the face, the parts that encircle the mouth and 

 sense-capsules and that form the basket-work of the branchial appa- 

 ratus — these appear later. 



For details of what can be seen in the growing skull of a Salamandrian, 

 I must refer to the main paper. Here I may remark that, while the 

 development of the Batrachian skull seems strongly to favour the doc- 

 trine of t}iQ facial nature of the "^ro-notochordal " bands (" trabeculae "), 

 the study of the Urodelous type suggests that they are truly hasal 

 (hasicranial). 



In front they are evidently facial, or belong to the visceral-arch series, 

 and are axial to the premaxillary arch ; but in the internasal and inter- 

 orbital regions they are very probably mere continuations of the para- 

 chordal tract of mesoblast which in the trunk gives rise to the bodies 

 of the vertebrae. 



