44 PEOF. G. B. HOWES AND ME. H. H. SWINNEETON ON THE 



at Q (text-fig. 9) there are four foramina and five roots. It is certain that, during 

 ontogeny, two roots at least disappear, and further investigation is necessary to render 

 perfectly clear the detailed nature of those which remain a . 



We may most fittingly refer here to a series of ligamentous supports which, so far as 

 we can ascertain, are new. We first detected them at Stage Q in the occipital region. 

 At S each is very conspicuous, and with its fellow of the opposite side is closely 

 applied to the under face of the medulla oblongata. It is strengthened at its bases by 

 ingrowths of the occipital cartilage (l.s.' 9 text-figs. 7, 8, & 10), and in the floor of the 

 neural canal it enters into a loose fibrous connection with the dura mater (cf. fig. 10). 

 This remarkable structure is repeated seginentally throughout the trunk-region of the 

 vertebral column, and we propose to term it the sustentacular ligament. 



The supra-occipital calls for no special comment. 



The opisthotic (op., PI. IV. fig. 9), which Siebenrock has proved 2 , to be an inde- 

 pendent element (the " paroccipital " of Owen), is at this stage remote from both the 

 prootic and exoccipital, and the ossific centre of the former is now beginning to 

 monopolize the feebly- developed cartilaginous parotic process (p-p-}, which appeared 

 at Stage Q and is still short. The prootic (" otosphenoid," auct.) (pro., figs. 8 & 10) 

 is a simple bone, and its processus anterius inferior, so characteristic of the adult 3 , has 

 not yet appeared, nor has ossification yet encroached upon the processus anterius 

 superior, which, still hyaline, forms (cf. fig. 8, c j .) not only at this, but at Stages R 

 and S, a formidable projection. 



The Mandihdar Arch. This is first differentiated at Stage P, in the form of a 

 minutely procartilaginous mass having a body or quadrate portion (PL III. fig. 1, q.) 

 bearing three outgrowths : an antero-dorsal or pterygoid (p.g. 1 ), a postero-dorsal the 

 future epipterygoid (columella), epg., and an antero-ventral, much the stronger of the 

 three, the Meckelian bar (mk.). 



The main body or quadrate mass, at this stage somewhat squarish, is disposed 

 parallel with the parachordal (pc.), and throughout the later stages, at which the 

 rotation of the latter and the straightening out of the basis cranii already referred to 

 are effected, it retains this relationship, i. e., the absolutely distinctive characters of the 

 whole arch are present in procartilage. Chondrification is seen to be setting in 

 independently in its body and the Meckelian bar, and even at this early stage the 

 latter is connected with its fellow symphysially by a thin procartilaginous tract. At 

 Q advancing chondrification of the Meckel's cartilage invades the symphysial tract, 

 and the whole cartilaginous lower jaw becomes for the time a single hyaline mass. 

 It remains such until Stage T, when the definitive symphysis begins to appear. 



1 Cf. Peter on Ichthyophis (footnote, infra p. 70). 



a Siebenrock, F. : 1893, p. 304, English Transl. 



3 Cf. Siebenrock, F. : Transl. cit. p. 307, & fig. 14, pi. 10. & Bruhl, < Zootomie aller Thier Kl.' pi. 149. fig. 5. 



