SfcOO DEVELOPMENT OF THE MARSUPIAL SKULL, 



of this latter lies nerve vii., with its geniculate ganglion. 

 Meckel's cartilage is at this point imperfectly chondrified. 



Section H(fig.9) is through the posterior part of the capsule. 

 Here the basal cartilage is broadening out but is still flat. The 

 capsule is of large size, and in its upper part is seen the expanded 

 labyrinth with, on the outer and upper sides, portions of the 

 semicircular canals, and, at the upper and inner part, the base of 

 the recessus labyrinthi. 



Trichosurus vulpecula. Stage ii. 



This Stage represents the degree of development attained by 

 the Phalanger at birth, when the embryo measures 14 mm. in 

 greatest length. The skull is now well chondrified, and the bones 

 of the jaws are ossified. In general structure it is as simple as 

 in the earlier stage, but a number of the elements can now be 

 made out with greater clearness, while some previously distinct 

 are now fused with others. 



The nasal capsule is still a simple, elongated, cartilaginous 

 cylinder with three openings in it — an anterior external for the 

 anterior nostril, a posterior superior for the olfactory nerves, and 

 an inferior opening for the internal nasal opening. The nostril 

 opens directly outwards, and is completely surrounded by carti- 

 lage above, in front, and behind. Inferiorly the nostril is 

 imperfectly floored by a cartilaginous flap which passes forward 

 from the base of the alinasal. There is no fenestra in the septum. 

 The paraseptal cartilage is still complete, and runs by the side of 

 the base of the nasal septum to the back of the nasal cavity, 

 where it is joined by a broad bar to the base of the aliuasal. 



The upper and outer part of the alinasal is continued back- 

 wards as a broad cartilaginous bar lying by the side of the brain, 

 and forming a support to it, and joins the upper part of the 

 auditory capsule. This is the orbitosphenoid cartilage. In front, 

 besides joining the alinasal, it also joins the basal cartilage, and 

 a small fenestra is formed between its two attachments. 



The basal cartilage, on passing back from the nasal region, 

 broadens out gradually, and, on reaching the plane of the pitui- 



