316 EMBKYOLOGY OF THE LOWER.VERTEBRATES CH. 



continuous with the trabecula in front and with the acrochordal 

 cartilage behind. The connective tissue between the anterior ends 

 of the trabeculae gradually chondrifies in continuity with them in 

 both Chick and Duck (Sonies). In the Kestrel Suschkin found a, 

 for a time independent, intertrabecular plate of cartilage in this 

 position (Fig. 158, itr). This intertrabecular tract of cartilage 

 serves to bound anteriorly the fontanelle (Fig. 157, D, p.f) in which 

 the pituitary body lies and through which pass the two internal 

 carotid arteries. Posteriorly this fontanelle is demarcated from the 

 posterior basicranial fontanelle by the acrochordal cartilage later the 

 posterior boundary of the sella turcica. It appears to be character- 



tr 



a.c 



n.a. 



a.c. 



FIG. 158. Early stage in the development of the chondrocranium of the Kestrel (Tinnuncid'u^ 

 alaudarius). A, side view; B, dorsal view. (After Suschkin, 1899.) 



o.c, auditory capsule ; itr, intertrabecular cartilage ; n.a, neural arches ; tr, trabecula ; - 

 III, foramen for oculomotor nerve. 



istic of Birds that this dorsum sellae undergoes a considerable 

 amount of reduction during later development. 



In Chick embryos of about 12 mm. a patch of cartilage has made 

 its appearance external to the otocyst between the lateral and the 

 superior (anterior) semicircular canals which gradually spreads 

 forming the external wall of the auditory capsule and closely 

 moulded to the surface of the canals. This periotic carlil.im- 

 ir7, K, <'.c) is for a time separated ly a wide g.-ip from the 

 basilar plate but this gap gradually becomes UK >re and more encroached 

 upon until reduced to a narrow fissure through wliie.h cranial 

 nerves IX, X, ami XI fiml lln-ir exit. Apart from this lissmv the 

 basal and periotic cartilages become continuous. As the wall of the 

 auditory capsule extends dorsally it remains incomplete at two 



points \\hriv p.-rfonil.-d |,y i|,,. |-'ari;il and Auditory mrvrs. 



The roof of tin- ( ImndiMi r.niium is represented by a quitr 



