ix.] DEVELOPMENT. 145 



with its extremities rather upturned and thickened. The 

 posterior cornu (th) consists of a single, nearly straight, 

 compressed bone, the thyrohyal, articulated inferiorly with 

 the outer end of the basihyal, just below the attachment 

 of the ceratohyal, and truncated at its superior extremity, 

 to which the thyroid cartilage of the larynx is suspended. 



Development of the Skull. For a detailed and beautifully 

 illustrated account of the early development of the Mam- 

 malian skull, we must refer to Professor Parker's monograph 

 "On the Structure and Development of the Skull in the 

 Pig' : (Philosophical Transactions, 1874). As however the 

 prevailing views regarding the morphology of the skull have 

 been considerably modified even since the publication of 

 that memoir, we take the following abstract from Balfour's 

 Treatise on Comparative Embryology, 1881. 



The primitive cartilaginous cranium is formed by a 

 differentiation within the membranous cranium, and is 

 composed of the following parts : 



(i.) A pair of cartilaginous plates on each side of the 

 anterior extremity of the notochord, known as the para- 

 chordals (Fig. 51, pa.ch.\ The continuous plate, formed 

 by them and the notochord, is known as the basilar or 

 basicranial plate. 



(2.) A pair of bars forming the floor for the fore-brain, 

 known as the trabeculae (tr) ; these are continued forward 

 from the parachordals. They meet behind and embrace the 

 front end of the notochord ; and after separating for some 

 distance bend in again in such a way as to inclose the 

 pituitary body (py). In front of this they coalesce, and 

 together with the olfactory capsules (pi) give rise to the pre- 

 sphenoidal and ethmoidal regions of the cranium (Fig. 47, I.). 



(3.) The cartilaginous capsules of the sense organs. Of 

 these the auditory and olfactory capsules unite more or less 



