DE"\^LOPMENT OF THE SKULL IN THE OSTEICH TRIBE. 115 



bones ; for although the bone-layer of a cartilaginous plate or rod may never affect the 

 cartilage-cells at all, yet it is always inside the perichondrium. The splint bones may 

 enfold persistent cartilage, or this pith may be absorbed ; whilst in other cases we find 

 the secondary bone rambling away far from the cartilage which served at first as its 

 model, and running up and down any aponeurotic tract that may be available for it. 

 Splint bones are not called secondary because of any lateness in their appearance — they 

 are the first to appear ; but cartilage is the first, or embryonic skeleton, and in certain 

 low Vertebrata never takes on a calcareous condition. 



One important difficulty turns up in this part of our research, and that arises from 

 the fact that certain parts of the face, viz. the pterygo-palatine arcade, ossify as early 

 as the earliest splint bones, whilst the tissues of the embryo are still simply cellular. 

 Careful attention to this matter has led me to see that the difficulty is not at all insu- 

 perable, and that it does not in the least affect the distinction between the two classes 

 of ossifications. I shall explain this more perfectly in subsequent papers ; yet it seemed 

 necessary to me to premise my description of the struthious skull with some remarks on 

 the histology of the ossifying structures. 



I must remark, further, that in studying Rathke's beautiful researches (by the 

 help of Professor Huxley's translations), 1 have been often confused by the use of 

 the term " cartilage," both for condensed tracts of fibrous tissue, and for the true, or 

 hyaline cartilage. The loose use of this term in Rathxe's most invaluable works 

 makes it necessary that those who follow him should repeat his observations in every 

 possible instance. Holding this one point of weakness in memory, no better guide can 

 be had. 



I have also been led to differ very considerably from this truly great anatomist in the 

 determination of the homologous bones in the various classes. Professor Huxley's 

 researches have shed much light upon this matter, but I find still further correction 

 needed. 



Returning to the earliest of the struthious skulls, we find that the hyaline cartilage 

 which has been formed in the primordial membranous brain-case is perfectly con- 

 tinuous (Plate VII.). And this is not only the case with the proper cranial chamber, 

 but parts truly facial are in nowise differentiated from those which belong to the 

 cranium proper. Thus the auditory capsules are continuous with the occipital carti- 

 lage behind and below, and with the posterior sphenoidal region in front and below. 

 The anterior sphenoidal cartilage passes (in front) continuously into the upper parts of 

 the lateral ethmoids above, and into the great middle ethmoidal plate below (Plate VII. 

 figs. 1 & 2, eth., O.S., p.e., p.s.). 



These two pairs of sense-capsules, the olfactory and the auditory, are not the only 

 parts which are continuous, as cartilage, with the cranium; but the primordial, or 

 fundamental part of the intermaxillary apparatus (Plate VII. figs. 1 & 2, p.n.) has not 

 shown the least disposition to segment itself from the vertical ethmoid, and indeed it 

 is one with the continuous orbito-nasal septum. On the other hand, the palatine has 



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