EMBRYO AND EMBRYONIC MEMBRANES 



215 



at the side by the maxillary processes, and in front by the naso- 

 frontal process, which is a broad projection below the cerebral 

 hemispheres overhanging the mouth. On each side of the naso- 

 frontal process are the olfactory pits, the cavities of which are 

 continuous with the oral cavity. Lateral to the olfactory pits 

 are the external nasal processes, abutting against the eye and 

 separated from the maxillary process by the lachrymal groove. 

 The portion of the naso-frontal process bounding the olfactory 

 pits on the median sides may be called the internal nasal process. 



Fig. 125. — Head of an embryo of 4 days' in- 

 cubation, from the oral surface (N. L. 6 

 mm.) 



Ep., Epiphysis. Hem., Cerebral hemi- 

 sphere. Hy., Hyoidarch. 1. nas. pr., Lateral 

 nasal process. Md., Mandibular arch. Mx., 

 Maxillary process, nas.fr., Naso-frontal pro- 

 cess. Olf., Olfactory pit. Or., Oral cavity. 

 Ph., Pharynx, v. A. 3, Third visceral arch. 



During the fourth and fifth days a fusion is gradually formed 

 between the internal nasal process on the one hand, and the 

 external nasal and maxillary processes on the other (Fig. 126), 

 thus forming a bridge across the open mouth of the olfactory 

 pits and dividing the openings in two parts, one within the oral 

 cavity, which becomes the internal nares or choanse, and one 

 without, which becomes the external nares or nostrils. During 

 the same time the whole naso-frontal process begins to project 



