The Clioiidroeranium of au Embryo Pig. 193 



almost the character of a horizontal septum, to its front end, where 

 it passes insensibly into the hinder end of the nasal septum. In 

 this it differs markedly from its homolog in Semnopithecus, Homo 

 and Lepus, in which the height is nearly uniform. In Talpa the 

 posterior end of the nasal septum rises almost perpendicularly above 

 the floor of the cavum cranii, whereas in Sus the crista galli runs 

 forward horizontally. The explanation for this difference lies in 

 the marked ventral flexion, which the facial part of the skull in 

 Sus has undergone in relation to the neural portion (PI. III). 



The basal portion of the regio orbitotemporalis, which lies in front 

 of the sella turcica, is higher than wide and so forms a vertical 

 plate, a true interorhital septum. Near its anterior end, where it 

 joins the nasal septum, it is 21/2 times as high as broad (Fig. 6). 



In Semnopithecus and Macacus, Fischer (1903) also found an 

 interorhital septum. This very primitive character of the mammals 

 is also shared by the reptiles and birds, which possess this tropibasic 

 or keeled type of skull. As Gaupp has plainly shown (1900), this 

 keel was developed as a result of the increased size of the eyes forcing 

 the lateral walls of the brain-case more and more together till only a 

 vertical plate remained between the eyes. In the mammals this keel 

 has been obscured by the subsequent development of other parts, 

 namely: (1) the increase in the size of the brain, which presses 

 upon it from above and from the rear; (2) the formation of the 

 secondary palate and the backward prolongation of the narial tube 

 (ductus nasopharyngeus), which thus limit its ventral extent; and 

 (3) the backward shifting of the posterior wall of the nasal capsule in 

 connection with the development of the ethmoturbinals, thus in- 

 creasing the extent of the nasal septum at the expense of the inter- 

 orhital septum. 



The lateral parts or alse of the orbitotemporal region are dis- 

 similar in size, shape, direction and location. Posteriorly is the small 

 single-rooted ala temporalis, extending directly outward from the 

 side of the basal cartilage and lying ventral to the general wall of 

 the cranial cavity. Anteriorly is the double-rooted ala orhitalis, ex- 

 tending diagonally outward and upward and expanding into a broad 

 plate, which forms the antero-lateral wall of the brain cavity. The 



