402 
MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
ear-capsule {cm .) ; the space is more than half an oval, the floor sinking downwards ; 
here, thin, ectosteal laminae are forming the basi- and exoccipital bones {bo., eo.). 
The most projecting part of the ear-capsule {cm.) contains the ampulla of the posterior 
canal. 
Fifth Stage. Embryo Snakes, 4-| to 5 1- inches long ; a few days before hatching. 
Within a short time the development of the bony centres has been so great as to 
give, in many cases, the form, to each ossification, which is to be permanent. 
This is especially the case in those bones which have no cartilaginous groundwork; 
those that have are still separated from each other by large tracts of cartilage (Plate 31, 
figs. 1-2 ; in fig. 1 the upper part of the skull has been cut away). 
When a strong osteo- cranium, like that of a Snake, has to be formed, the ossifica- 
tions soon blot out old morphological landmarks. It is indifferent to bone-tissue what 
variety of the connective-tissue series it shall get its matrix from : it is as if the clay 
and the straw laid out on the original ground-plan should indifferently be made into 
bricks and tiling. 
We see here that the posterior basal fontanelle {p.b.c.f) has grown to its utmost 
size ; but the apex of the egg-like outline is taken off’ by the extension of the 
basioccipital (bo.) into the membrane. That ossification is very large and heart- 
shaped, the “ cephalostyle ” lying on its concave or upper surface ; it extends its bony 
growth into the substance of the bilobate condyle {oc.c.). 
The exoccipitals (eo.) are extending, but the cliondrocranium keeps growing also ; 
therefore there is still one large tract unossified between the median and lateral bony 
centres. 
The lateral bones are riddled with holes, and these have added to them others that 
extend the series to the pituitary space. 
Posterio-externally, we have the “posterior condyloid foramen” ( p.c.f, '.); then within 
and in front of that the anterior (12), for the hypoglossal nerve, which now, for the 
first time in our ascent, has become a cranial nerve. 
For the substance of the parachordal cartilages now gathers to the mid-line, to form 
the emarginate single condyle ; this double tract, in many Urocleles, forms an interca- 
lary vertebra — the odontoid (“ Skull of Urodeles,” p. 575). 
Near to the ear-capsule and its fenestrse we see the foramina for the vagus (10) and 
glossopharyngeal (9) ; these holes have a bony tract between them, as in most 
Batracliia. 
The super- occipital centre is ossifying ; this, however, will be best seen in the next 
stage (figs. 3, 4, 6, so.). 
The two films of bone that begin outside the carotid opening are now one large 
bone : the basisphenoid ( bs .). This bone now occupies the roots of the trabeculae, but 
stops in front, opposite the end of the parasphenoid {pa.s.), and at the beginning of 
the narrow interorbital region. 
