440 



the occipital foramen, there arises a very small semilunar bony plate, whose 

 concave edge or excavation is directed forwards j thereupon the bony 

 substance shoots from this edge further and further forwards, until at 

 length the bony plate has the form of the ace of hearts. Its base 

 borders the fontanelle in the base of the skull, which lies under the 

 anterior half of the third cerebral vesicle, while its point is contiguous to 

 the occipital foramen ; for the most part it is very thin, and only its axis 

 (and next to this its whole posterior margin) is distinguished by a greater 

 thickness. The cephalic part of the chorda can be recognized in the axis 

 of this bony plate up to the following period. It passes from the posterior 

 to the anterior end of the bony plate, where it is lost, and is so invested 

 by the bony substance of the plate, that a smaller portion of the latter 

 lies on the upper side of the chorda, a larger portion beneath it. On this 

 account it forms, on the upper side of the plate, a longitudinal ridge, 

 which subsequently becomes imperceptible by the aggregation of matter 

 at the sides. On one occasion, however, I saw, in an embryo which was 

 almost at term, a similarly formed and sized bony cone, which, through 

 almost its entire length, appeared merely to lie on the body of the basi- 

 occipital, since it had only coalesced with it below." 



The nucleus and sheath of the cephalic part of the chorda become 

 gradually broken up and the last trace of them eradicated, as the ossifica- 

 tion of the basioccipital 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 j 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 ethmo- 

 vomerine 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 still 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 rudi- 

 mentary cochlea), and developes into a moderately long, blunt, and hollow 

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



* 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 inseparable sheath of bony matter, just in the same way as the 

 ' urostyle ' is developed in the tail. 



