514 



EMBRYOLOGY. 



the front end of the transversely located maxillary processes, from 

 which they are delimited externally by the lachrymal grooves. 



On the median wall of the nasal pit there exists a special small 

 depression, which was first found by DURSY in mammalian embryos, 

 and which is also observable in human embryos at a very early stage 

 (His). It is the fundament of JACOBSON'S organ, which afterwards 

 makes its way into the septum of the nose. It receives from the 

 olfactory nerve a special branch, which is indeed of remarkable size 

 in embryos. 



The stage with the nasal groove exists as the permanent condition in many 

 Selachians. In these cases the deep nasal pits, which are enclosed in a car- 

 tilaginous capsule, and the mucous membrane of which is raised up into 



numerous parallel folds, 

 lie on the under surface 

 of the elongated snout or 

 rostrum. Deep grooves, 

 which are bounded by 

 folds of the skin contain- 

 ing muscles, and which 

 can be closed as if by 

 valves, lead to the front 

 margin of the mouth at 

 some distance from its 

 angle. 



Fig. 289. Roof of the oral cavity of a human embryo with the 

 fundaments of the palatal processes, after His. Magnified 



The next stage, 

 which in human em- 

 bryos is reached in 

 the second half of 

 the second month, 



exhibits the organ of smell converted into two canals, which have 

 been produced by the fusion of the margins of the two grooves, 

 especially that of the inner nasal process with the maxillary process, 

 which advances toward the median plane. The canals now possess 

 two openings, the external and the internal nasal orifice (fig. 289) or 

 the nares. The two external nares lie only a little above the border 

 of the mouth-opening; the internal, in the roof of the primitive oral 

 cavity, on account of which they have been named by DURSY the 

 primitive palatal clefts. They are located far forward, only a little 

 removed from the edge of the mouth, a position which they retain 

 permanently in the case of the Dipnoi and Amphibia. At first 

 round, they afterwards become elongated and assume the form of 

 a fissure running from in front backward. 



With the metamorphosis of the organ of smell into a canal leading 



