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., Mandibulararch. 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 choane, and one 
without, which becomes the external nares or nostrils. During 
the same time the whole naso-frontal process begins to project 
