540 



EMBRYOLOGY OF THE CHICK 



floor for the mid and hind-brains. These plates then fuse both dorsally 

 and ventrally around the notochord, and consequently enclose it. The 

 fused plate is then known as the basilar plate, forming the floor of the 

 hinder portion of the skull. 



The auditory capsules which enclose the auditory organs form and 

 fuse to the sides of the basilar plate. It is from growths of the basilar 

 plate and the auditory capsules that the floor and occipital portions of 

 the skull are formed. 



The anterior portion of the skull is formed from two slender rods 

 lying cephalad to the notochord, but which are in connection with the 

 parachordal plates. These rods are known as trabeculae cranii. 



The pituitary body lies between these trabeculae cranii, so that in 

 fusing as they now do to form the ethmoid plate, the pituitary body 



comes to He in the position where it will 

 be found when we study Comparative An- 

 atomy. 



The ethmoid plate (Fig. 311) extends 

 cephalad to the tip of the beak, and fuses 

 anteriorly with the olfactory capsules. 



The interorbital septum develops as a 



Profile view F Tf 2-day chick-skuii. large vertical plate from the dorsal surface 

 as. aiisphenoid; d, dentary ; e, of the ethmoid plate, along the whole median 



ethmoid; /, frontal; I, prefrontal ; ** 



mx, maxillary; n, nasal; ol, lateral hne. It IS quite COmniOn tO Speak of Car- 



occipital ; os, superior occipital ; pa, ... 1 i i r A 1 



palatine; pm, premaxiiiary ; pt, tilagmous and membranous bones of the 



skull. This means only that some of the 

 bones there formed (in fact, all these we 

 have just been describing) were first pre- 

 formed in cartilage, and then became bone, 

 while the membranous bones were first cartilage, and then, by being 

 placed where there was considerable stretching, they became quite thin 

 membranes before they finally ossified. 



The membranous bones form the roof of the skull, such, for exam- 

 ple, as the parietals, frontals, etc. 



THE VISCERAL SKULL 



It will be remembered that the first visceral arch was also called the 

 mandibular arch, because it is from this arch that the mandible or lower 

 jaw is formed, and that the second visceral arch was known as the hyoid 

 arch, because it is from this that the hyoid bone, or cartilage which sup- 

 ports the tongue, has developed. The parts of the skull which are thus 

 developed from the visceral arches form the visceral skull. 



THE HEART 



It is during the fifth day that the interventricular septum is almost 

 completed, fusing with the posterior edge of the septum which now 





portion of skull just becoming con- 



verted into bone. (Cartilage is 



stippled.) (After Boas.) 



