316 EMBRYOLOGY OF THE LOWEE VEETEBEATES 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 (Eig. 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- 



Fig. 158. — Early stage in the development of the choudrocranium of the Kestrel ( Tinnunculus 

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



ax, 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 cartilage 

 (Fig. 157, E, a.c) is for a time separated by a wide gap from the 

 basilar plate but this gap gradually becomes more and more encroached 

 upon until reduced to a narrow fissure through which cranial 

 nerves IX, X, and XI find their exit. Apart from this fissure the 

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

 auditory capsule extends dorsally it remains incomplete at two 

 points where perforated by the Facial and Auditory nerves. 



The roof of the chondrocranium is represented by a quite 



