281 



the oral cavity there is developed a special region of ossification, this 

 furnishes a thin bony lamella which is preserved in many mammals 

 as a special skeletal element (os pterygoideum) lying on the pterygoid 

 process of the sphenoidale. In man it early fuses with the sphenoid, 

 notwithstanding it has an entirely ditferent origin from the latter" 

 (Hertwig — trans, by Mark from 3rd German edition, p. 620). 



If this internal pterygoid plate be a "covering" bone it is somewhat 

 remarkable that it should ossify so late as the fourth month of fetal 

 life, as other covering bones e. g. the dentary, the processus gracilis 

 etc., appear very early. It was with this thought in mind that this 

 investigation was undertaken. 



Coronal and oblique sections were made of heads of embryos ranging 

 in age from 10 — 11 weeks to the latter part of the 3rd month. These 

 sections were cut and mounted serially either after paraffin or 

 celloidin infiltration and embedding. In passing, I think it may be of 

 use to say that for the older material celloidin was in most cases the 

 more satisfactory medium as there was less liability on the part of 

 the bone to tear out of the surrounding tissue, then, too, as it saves 

 a great deal of time and annoyance, I may mention that the sections 

 were mounted by the "warm water" method, but before floating them 

 into position on the slide, the slide had rubbed over it with the 

 finger tip, a thin layer of albumen-glycerine fixative. This allows the 

 water to at once cover the slide, which it will not otherwise do, if the 

 fixative be not used, unless the slide is very clean. I dare say glycerine 

 alone would do just as well, but I have not tried it. 



The sections were cut with Reichert's "New inclined plane 

 microtome" of thicknesses varying from 10 f.i to 20 [^i. Drawings were 

 made of selected sections, and some of them are presented here. 



Whilst examining the internal pterygoid plate and its hamular process 

 I was struck with the appearance presented by the external pterygoid 

 plate. It, though universally described as an otfshoot of the ali- 

 sphenoid and therefore presumably cartilaginous, will be seen in the 

 various figures to be not so, for, like the internal pterygoid plate, it 

 is almost, if not wholly, 'membranous' and from that, is transformed 

 into bone, without chondrification. 



Let us now examine the drawings of sections. 



figure 1. This a drawing of a coronal section of an eleven weeks 

 fetal head, and the section passes through, the tonsil (T), the internal 

 pterygoid plate at its junction with the hamular process I.P.P., and 

 the external pterygoid plate E.P.P. The hamular process H.P. is 

 seen here unossified, with the tendon of the tensor palati on its outer 

 side. To the outer side of this tendon is the internal pterygoid muscle 



