IX.] 



THE ETHMO-PRESPHENOID PLATE. 



231 



posterior clinoid walls. But it is in front of the foremost of 

 these that the most noticeable changes take place. From 

 the mid-line of the coalesced trabecule there rises up a 

 high ridge, the ethmo-presphenoid plate. This plate is at first 

 liighest behind. In front and below it sends out a process, 

 the prenasal cartilage, which forms the cartilaginous basis 

 on which the premaxillary region is moulded. 



12. Development continues to be very rapid in these 

 parts; and on the seventh day the anterior end of the 

 ethmo-presphenoid plate (Fig. 70, eth and ps) becomes its 

 highest point and forms the retral spike of the ethmoid 

 (Fig. 70, eth). 



Fig. 70. 



tcln 



mk. hh/ b 



or 



cbr 



Side View of the Cartilaginous Cranium of a Fowl on the Seventh 



Day of Incubation. (From Parker.) 



pn prenasal cartilage. ahi alinasal cartilage. ale aliethmoid ; immediately 

 below this is the aliseptal cartilage, eth ethmoid, pp pars plana, ps pre- 

 sphenoid. pa palatine, pg pterygoid, z optic nerve, as alisphenoid. 

 q quadrate, st stapes, fr fenestra rotundum. hso horizontal semicircular 

 canal, psc posterior vertical semicircular canal : both the anterior and the 

 posterior semicircular canals are seen shining through the cartilage. 

 so supraoccipital. eo exoccipital. oc occipital condyle, nc notochord. 

 ink Meckel's cartilage, ch cerato-hyal. bh basi-hyal. cbr and ebr cerato- 

 branchial. bbr basibranchial. 



The prenasal cartilage (Fig. 70, pn) still points down- 

 wards, and by this time are formed the alinasal cartilages 

 (Fig. 70, aln) developed from the trabecular horns, and the 

 aliseptal cartilages which enclose the inferior turbinate 

 (Fig. 70). 



The basisphenoid grows outwards on each side to form 



