SKELETON AKD LAETNX OF XENOPFS AKD PIPA. 107 



development conforms to the general rule in so far that the thyro- 

 hyals arise from the hinder part of the hypobranchial plate, and 

 at the proximal end of the fourth branchial arch. 



The thickening of the anterior portion of the hypobranchial 

 plate is the developing ala of the hyobranchial skeleton of the 

 adult. It is situated, strictly speaking, anteriorly to the level of 

 the first branchial arch, from which it is already beginning to 

 separate by the formation of the notch shown atf in fig. 6. The 

 bar of cartilage lying in the median line between the two foramina 

 will become the isthmus (PI. 9. fig. 1, t) which connects the 

 anterior and the posterior or laryngeal parts of the basal plate of 

 the adult. The hyoid cornua or ceratohyals are much thinner 

 than in Stage II. and are slightly longer. They are of approxi- 

 mately uniform thickness all along, except at their median 

 extremities, where they are reduced. They are delicately curved, 

 the anterior margin being convex and the posterior concave. The 

 space between the hyoid and first branchial arches has increased 

 in size, and the indentation between the two hyoidean cornua is 

 no longer V-shaped but semicircular, with a slight tendency to 

 squareness. The larynx, which has increased but little in size, 

 now overlaps the developing thyrohyals *. 



Stage IV. (PI. 11. fig. 7.) 

 The difficulty experienced in dissecting out the branchial arches 

 in the last stage, owing to the looseness and softness of the dis- 

 integrating cartilage, here reaches a maximum ; and it is only 

 with the very greatest care that the arches can be recognized at all. 

 All four have severed their connexion with the hypobranchial 

 cartilage, and the epibranchial or commissural cartilages at their 

 distal ends have completely disappeared. The part of the hypo- 

 branchial cartilage postero-external to the thyroid foramen and 

 between the developing ala and thyrohyal is not yet entirely 

 absorbed, but the tissue in this position is of a very loose 

 character. The most instructive lesson that this stage teaches 

 is that the ala of the adult skeleton is a purely hypobranchial 

 derivative and that it is morphologically anterior to the first 

 branchial arch. Parker's interpretation, therefore, of the ala 

 as the confluent first and second branchial arches of the larval 

 skeleton is no longer tenable. 



* In order to avoid confusion, it has been omitted in figs. 6, 7, and 8 ; its- 

 position in fig. 9 is indicated by the dotted line. 



