242 ON THE STRUCTURE OF THE SKULL. 



ossify. At a little distance from the occipital foramen, there arises 

 a very small semilunar bony plate, the concave edge or excava- 

 tion of which is directed forwards ; 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 notochord can be recognised 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 osseous substance of the plate, that a 

 smaller portion of the latter lies on the upper side of the notochord, 

 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 had 

 almost reached its full 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 notochord 

 become gradually broken up and the last trace of them eradicated, 

 as the ossification of the basi-occipital proceeds, like the nucleus 

 and sheath of the rest of the notochord wherever a vertebral body 

 is developed.* 



The articular condyle is not yet formed. The ex-occipitals 

 ossify through their whole length and breadth. 



The body of the basi-sphenoid is formed between the above- 

 mentioned posterior fontanelle of the basis cranii and the 

 pituitary space, therefore far from the cephalic part of the noto- 

 chord. It ossifies by two lateral centres, each of which forms 



* 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. 



