592 ON THE THEORY OF THE VERTEBRATE SKULL 



occipital proceeds, like the nucleus and sheath of the rest of the chorda wherever a. 

 vertebral body is developed. ^ 



The articular condyle is not yet formed. The exoccipitals ossify through their- 

 whole length and breadth. 



The body of the basisphenoid is formed between the above-mentioned posterior 

 fontanelle of the basis cranii and the pituitary space, ' therefore far from the- 

 cephalic part of the chorda.' It ossifies by two lateral centres, each of which- 

 forms a ring round the carotid canal. The alisphenoids ossify in their whole 

 length and breadth ; the orbitosphenoid only slightly, and the presphenoid not 

 at all. The premaxillary bone arises as an azygos triangular cartilage between 

 the cornua of the anterior ethmovomerine plate. It ossifies from a single centre. 



" The auditory capsule, or the future petrosal bone, may, even at the end of 

 this period, be readily separated from the other part of the cranial wall, and stilli 

 consists for the most part of cartilage. On the other hand, the triangular form,, 

 which it had before, is not inconsiderably altered, since it greatly elongates 

 forwards, and thus, as it were, thrusts its anterior angle further and further 

 forwards, and becomes more unequal-sided. At the lower edge, or the longer 

 side of it, about opposite to the upper angle, at the beginning of this (third) period, 

 or indeed somewhat earlier, a diverticulum of the auditory capsule begins to be- 

 formed (the rudimentary cochlea), and developes into a moderately long, blunt,, 

 and hollow appendage, whose end is directed downwards, inwards and backwards,, 

 and also consists of cartilage. Close above, and somewhat behind this append- 

 age, however, there appears, at about the same time, a small rounded depression, 

 in which the upper end of the auditory ossicle eventually rests ; and somewhat 

 later, an opening appears in this depression which corresponds with the fenestra 

 rotunda of man. Very much later, namely, towards the end of this period, the 

 auditory capsule begins to ossify. Ossification commences in a thin and moder- 

 ately long, hook-like process, which is sent forwards and inwards from the 

 lower hollow diverticulum of the cartilage, and unites with the basisphenoid. 

 From this point it passes upwards and backwards, and, for the present, extends, 

 so far that, at the end of this period, besides that process, the diverticulum in 

 question and about the anterior third of the auditory capsule itself, are ossified. 

 Later than at the point indicated, an ossific centre appears at the posterior edge 

 of the auditory capsule, where it abuts against the supra- and ex-occipitals, but 

 extends from hence by no means so far forward as to meet that from the other 

 point. The middle, larger part of the auditory capsule, therefore, for the present, 

 remains cartilaginous. 



" In the beginning of the fourth period, a third ossific centre arises in the upper 

 angle of the capsule, whereupon all three grow towards one another. But the mode 

 of enlargement and coalescence of these bony nuclei is very remarkable. They 

 do not unite with one another in such a manner as to form a continuous bony 

 capsule for the membranous part of the labyrinth, but are permanently separated 

 by cartilagino-membranous and very narrow symphyses. On the other hand, 

 one coalesces, in the most intimate manner, with that edge of the supraoccipital 

 which is nearest to it, so that even in the more advanced embryos, this bone 

 and it form a moderately long oblong plate, each end of which constitutes a 

 small, tolerably deep, and irregularly-formed shell, containing a part of the 

 anterior or upper semicircular canal. The second bony centre becomes an- 



' In the stickleback it has appeared to me that the wall of the anterior conical termination 

 of the notochord in the basis cranii becomes ossified, or at any rate, invested by an in- 

 separable sheath of bony matter, just in the same way as the ' uro style ' is developed in the 

 tail. 



