Lateral Wall of the Cavum Nasi in Man. 617 



thus passively depressed. The folds are not complete but are 

 deficient inferiorly, and the thickening is most marked medially 

 and laterally. The comparatively excessive medial and lateral 

 thickening is thought by Kallius to be in anticipation of the later 

 medial and lateral nasal processes. The nasal plates or areas 

 thus become the olfactory or nasal pits, separated by a broad mass 

 of tissue — the fronto-nasal process or the Stirnfortsatz of German 

 embryologists. 



The pits are primitively more or less pyriform in shape. The 

 clubbed ends are directed somewhat medially and towards the 

 vertex of the head, and the inferior extremities are directed some- 

 what laterally and towards the primitive oral fossa — the stomo- 

 daeum. As the pits deepen they seem to separate the inferior 

 portion of the fronto-nasal process on both sides into medial and 

 lateral parts — the anlages of the medial and the lateral nasal 

 processes. During the latter part of the fourth week, or early in 

 the fifth week, the median portion of the fronto-nasal process 

 undergoes further differentiation into a median or unpaired part, 

 and two lateral or paired parts. The latter are globe-like and 

 will be spoken of as the medial nasal processes (Processi glob- 

 ulares, His), and they form the immediate medial boundaries of 

 the nasal pits. At the same time the lateral portions of the fronto- 

 nasal projection grow caudally and form the lateral nasal pro- 

 cesses — the immediate lateral boundaries of the nasal pits, or the 

 primitive lateral .nasal walls. 



Up to this time the nasal pits are not closed in interiorly, but 

 communicate freely with the primitive oral fossa. In other words, 

 the nasal pits and the oral fossa represent in a sense a common 

 cavity. At this stage of development the maxillary processes of 

 the first or mandibular arches grow ventrally and medially and 

 abut — later fuse — with the medial nasal processes (Processi 

 globulares). This fusion closes in the superior boundary of the 

 primitive oral cleft, and at the same time shuts off the path of 

 communication between the nasal pits and the oral cavity, i.e., 

 the nasal pits are closed in inferiorly (figs. 1 and 3). The coales- 

 cence of the maxillary processes with the medial nasal processes 

 forms the primitive inferior boundaries of the nasal pits; subse- 



