Development of. the Fifth Day 267 



auditory capsules, which enclose the auditory 

 organs. The floor and sides of the hinder 

 part of the skull are formed by the growth of 

 the basilar plate and the auditory capsules. 



The floor of the anterior end of the skull is 

 formed chiefly by the trabeculce cranii. These 

 are two rather short and slender rods that lie 

 in front of the notochord and are continu- 

 ous with the anterior ends of the parachordal 

 plates. They lie on each side of the pituitary 

 body, and fuse together in front of it, forming 

 the ethmoidal plate. The ethmoidal plate ex- 

 tends forwards to the tip of the beak, and is 

 fused, anteriorly, with the olfactory capsules. 

 The interorbital septum develops as a large 

 vertical plate from the dorsal surface of the 

 ethmoidal plate, along its whole median line. 



The above-described structures form the 

 cartilaginous skull, and, as incubation proceeds, 

 this cartilage is gradually changed to bone, 

 and forms the bones of the floor and sides of 

 the adult skull. These bones which are first out- 

 lined in cartilage are known as cartilage bones. 



The roof of the skull, the parietals, frontals, 

 etc., is chiefly made up of the so-called mem- 

 brane bones, that is, of bones that are not pre- 

 formed in cartilage. 



