236 THE DEVELOPMENT OF THE SKULL. [CHAP. 



the beginning of the fourth stages, two fenestras appear. 

 The hindermost of these separates the presphenoid from the 

 ethmoid ; but since the hole does not extend to the bottom 

 of the plate, the basi-sphenoid and the ethmoid are continu- 

 ous below. The other fenestra separates the ethmoid from 

 the nasal septum. 



The early condition of the ethmoido-nasal cartilage re- 

 sembles, as Parker points out, its permanent condition in the 

 Struthiadce, while the condition which it acquires during this 

 stage resembles its condition in Thiamine birds, seeming to 

 prove that the Gallinaceous birds passed through these two 

 stages before reaching the present condition. This plate of 

 cartilage remains during this stage quite unossified. The 

 aliethmoid, aliseptal, and alinasal cartilages which grow out 

 from it on each side, are now severally separated from each 

 other by slight grooves. 



18. The chief new ossifying centres which appear in the 

 primordial skull in this stage are : — 



(1) An ectosteal plate for the prootic. 



(2) Two ectosteal plates which appear in each ali- 

 sphenoid, one in the upper corner, and another immediately 

 above the foramen ovale. 



The centres which had begun during the last stages have 

 now spread considerably. 



The basioccipital is to a great extent ossified, but still 

 encloses the remains of the notochord. The occipital condyle 

 is however still unossified. Both the exoccipital and supra- 

 occipital plates are spreading rapidly, and the latter have 

 nearly reached the middle line. The quadrate is nearly 

 entirely ossified, but its upper and lower condyles and orbital 

 process are still cartilaginous. 



19. The rostrum is still separate from the ethmoid, 

 but has to a great extent coalesced with the basisphe- 

 noid. Between the posterior end of the pterygoid and the 

 rostrum, a plate of cartilage is interposed, called the basi- 

 pterygoid. 



The basitemporals have coalesced to a great extent with 

 the osseous outgrowths of the basisphenoid, but at the sides 

 a space is left between the two, through which the Eusta- 

 chian tubes pass. Between the basitemporals themselves, 

 where they have not coalesced, another space is left, which. 



