44 PROF. G. B. HOWES AND MR. H. H. SWINNERTON 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’. 
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 (J.s.’, text-figs. 7, 8, & 10), and in the floor of the 
neural canal it enters into a loose fibrous connection with the dura mater (¢/. fig. 10). 
This remarkable structure is repeated segmentally 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., Pl. IV. fig. 9), which Siebenrock has proved”, 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 *, has 
not yet appeared, nor has ossification yet encroached upon the processus anterius 
superior, which, still hyaline, forms (cf. fig. 8, ¢'.) not only at this, but at Stages R 
and S, a formidable projection. 
The Mandibular Arch.—This is first differentiated at Stage P, in the form of a 
minutely procartilaginous mass having a body or quadrate portion (PI. III. fig. 1, ¢.) 
bearing three outgrowths: an antero-dorsal or pterygoid (p.g.'), 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 (pe.), 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, @. ¢., 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. 
* Of. Peter on Ichthyophis (footnote, infra p. 70). 
* Siebenrock, F.: 1893, p. 304, English Transl. 
* Of. Siebenrock, F.: Transl. cit. p. 307, & fig. 14, pl. 10, & Briihl, ‘ Zootomie aller Thier Kl.’ pl. 149. fig. 5. 
