500 MAMMALIAN ANATOMY 



the posterior lateral fontanelle is reduced to a small quadrangular 

 cartilaginous area between the supraoccipital, exoccipital, parietal, and 

 petro-mastoid. 



The ectotympanic of the temporal is now a stout ring ; the ento- 

 tympanic is still cartilaginous. When the petrous is examined from 

 the interior of the cranium, the superior and posterior semicircular 

 canals are seen outlined in bone almost as plainly as in the drawing 

 from the metal cast of the labyrinth (Fig. 160). The aquseductus 

 vestibuli opens on the posterior surface at the anterior end of a deep 

 notch in the posterior border. 



The anterior lateral fontanelle is present ; in the specimen from 

 which Fig. 402 was drawn, it extended downward as a slit between 

 the squamous portion of the temporal and the orbitosphenoid. The 

 inner wall of the orbit exhibits the narrow band of membrane. The 

 external pterygoid fossae are divided by the pterygo-alisphenoidal 

 sutures. The true frontal sinuses are beginning as clefts in the 

 posterior wall of the external frontal sinus. 



The eight pieces of the lateral ethmoid are all more or less ossified; 

 they are united in front and below, but are separate above. The ossi- 

 fications in the lower six pieces have extended upward almost to the 

 cribriform plate, which, together with the mesethmoid, is still cartila- 

 ginous. The cartilage of the maxillo-turbinal has been almost entirely 

 replaced by bone. The ethmoid al scrolls are but slightly subdivided. 



The skulls at fifteen and twenty-three days stand in point of de- 

 velopment between the skulls at eleven and thirty days. At fifteen 

 days the entotympanic is ossified and the floor of the appendicular 

 fossa closed. 



The skull at thirty days (Fig. 403) shows a distinct advance in 

 development. 



The posterior lateral fontanelle has disappeared by the upward 

 growth of the mastoid part of the petro-mastoid. The squamous is 

 beginning to unite with the petrous and the tympanic. The ectotym- 

 panic ring has increased in thickness, hence the diameter of the ex- 

 ternal auditory meatus is reduced ; the entotympanic has ossified. On 

 the posterior surface of the petro-mastoid, the internal auditory meatus 

 has become smaller, the appendicular fossa shallow, and the posterior 

 semicircular canal less clearly defined. The aquaBductus vestibuli 

 opens on the posterior border. The posterior prolongation of the 



