38 PEOI<\ G. B. HOWES AND ME. H. H. SWINNEETON ON THE 



posterior portion, at this stage disposed at right angles to the anterior (its anterior 

 extremity being alone visible from above, as in fig. 2), is seen to similarly consist of a 

 basal or parachordal plate (par.) also bearing two pairs of outgrowths, viz. an anterior 

 (o.s.), to be hereafter termed the otosphenoidal plate, and a posterior, the auditory 

 capsule (c.au.), This posterior portion is hyaline only at the anterior and lateral 

 borders of the parachordal plate, the anterior chondrification extending into each of 

 the otosphenoidal tracts. The intracranial notochord, enclosed within the basal para- 

 chordal plate, terminates anteriorly in a minute freely-exposed apex (nc., fig. 2), and, 

 as already pointed out by Dendy for Stage N (99 a . p. 75), it takes a sinuous course. 



Dismissing for the present the sensory capsules, which at this stage are simple and 

 cup-shaped, it becomes necessary for comparison of later stages to deal fully with the 

 ethmo- and oto-sphenoidal plates. The former (es.) are large and wing-like and bear 

 no outgrowths. The latter are complex and wholly distinct from the trabeculse. 

 Each of these otosphenoidal plates is seen to be extending upwards and outwards, 

 backwards and inwards, and directly forwards. Two apertures are enclosed by it a 

 lower (j. Hi.), which transmits the third cranial nerve, and an upper (f. iv.), which 

 transmits the fourth 1 . The processes to which it is giving rise (indicative of differen- 

 tiation along definite lines) are five in number (os. 1 to os. 5 , PI. III. figs. 1, 2, 3). 



Before passing on to a later stage, we wish to emphasize the simple rod-like 

 condition and non-extension into the cranial wall of the trabeculee, which, except for 

 their connection with the procartilaginous basal ethmoid, are disposed serially with 

 the mandibular arch (j)g.mk.\ 



At Stage Q, despite the short advance in time upon P, remarkable progress is seen 

 to have been made in the formation more particularly of the cranium, nearly all the 

 definitive components of which are now recognizable. The trabeculae have now fused 

 posteriorly with the parachordal not by their ends as might be supposed, but by their 

 postero-internal borders, and they appear to us now to give rise to the basipterygoid 

 outgrowths (bs. 1 , PI. III. fig. 6). An accompanying upward rotation of the parachordal 

 (associated with cranial flexure) has brought about a consequent approximation of 

 the otosphenoidal plates to the trabeculae, with an accompanying fusion involving the 

 first and fifth otosphenoidal processes. Of these, the latter, passing downwards, 

 unites with the outer trabecular border, and thereby shuts off (f.j., fig. 4) a jugular 

 foramen ; the former, uniting with the inner trabecular border, encloses a passage (f.a.) 

 for the ophthalmic artery. 



More significant than the foreging is the change in position undergone by the 

 second and third otosphenoidal processes. Originally directed upwards, under the 

 combined influence of rotation and growth, they are now directed forwards, and, by 

 union with the posterior border of the sphenethmoidal plate, they have come to 

 bridge longitudinally the membranous lateral cranial wall. As the result of this, two 



1 Gaupp (1898, Berichte, p. 8) describes these nerves in the Lacertilian as related to a common fcnestra. 



