The Chondrocranium of an Embryo Pig. 203 



maxillaris posterior would separate the paries nasi from the planum 

 antorbitale. The same is true also of the mammals, although here 

 the planum antorbitale is usually oblique instead of transverse. 

 The change to the mammalian condition has probaljly been brought 

 about in the following manner: 



In the streptostylic reptiles the posterior part of the nasal capsule 

 is free from the septum. In the evolution into the nasal capsule 

 of the mammals, the posterior part of the capsule of the reptiles 

 has been expanded by the backward rotation of the posterior wall 

 (reptilian planum antorbitale), the pivot being the more solid 

 lateral side. This will be made more evident upon reference to 

 Fig. 11. This explains how a part of the intcrorbital septum in 

 the reptiles has been converted into the posterior portion of the nasal 

 septum in the mammals, and also how the ethmoturbinal region of 

 the latter has been derived. 



The solum nasi is as yet incomplete. The anterior connection 

 between the septum and the side wall by the lamina transversalis 

 anterior is narrow and the posterior part is quite free. The lamina 

 transversalis posterior is formed by an inrolling of the latero-vcntral 

 part of the planum antorbitale. At this stage the infolded ventnal 

 border forms a narrow horizontal shelf extending along the posterior 

 quarter of the capsule and is entirely free from the septum; more- 

 over, it has not yet been turned upward to form the imraseptal 

 lamella. From the lamina transversalis anterior there extends back- 

 ward the great fenestra hasalis, bounded medianly by the lower 

 border of the septum and laterally by the lower border of the side 

 wall. In connection with the lamina transversalis anterior, a strip 

 of precartilaginous tissue runs backward median to Jacobson's organ, 

 but it ends freely before reaching the lamina transversalis posterior. 

 This is the Aiilage of the cartilago paraseptalis. Later this whole 

 region is enclosed by membrane bones (maxilla, palatine, vomer) 

 so that it then lies within the nasal cavity. 



Forward, in the region of the external narial opening, the condi- 

 tions are more complex. The aperture of the nasal capsule (fenestra 

 narina, Gaupp), which is destined for the apertura nasalis externa, 

 looks ventrally and not forward or laterally as in many other mam- 



