420 ^Ir. R. Broom on the Proliferated Epithelium 



XLIX. — On the Significance of the Proliferated Epithelium in 

 the Foetal Mani?nalian Jaw. By R, Broom, M.B., B.Sc. 



If a transverse section be made through the anterior part of 

 the lower jaw of a seven-month human foetus, it will be 

 noticed that from the outer alveolar margin and about half of 

 the adjacent tooth-space arises a well-developed and projecting 

 gum, which is covered, especially on the inner side, by a 

 layer of epithelium many times thicker than that covering 

 the adjoining mucous membrane. On the inner side of this 

 epithelial covered ridge is the neck of the dental germ. 



On serial section it will be found that, though this ridge of 

 gum extends all K)und the alveolar margin, the thick coating 

 of epithelium is only met with in the region of the incisors 

 and canines, and that on reaching the first milk-molar the 

 epithelial armature of the gum differs little from that of the 

 general mucous membrane of the mouth. 



That this epithelial protection is directly connected with 

 the requirements of the young mammal in grasping and 

 retaining the nipple will readily suggest itself; but when one 

 finds that, within certain limits, the younger the foetus 

 examined the more is the epithelial ridge developed propor- 

 tionally, one is led to think that the proliferated epithelium 

 has a deeper significance than merely the strengthening of the 

 gum during the short period of sucking. 



Even in the human foetus of ten weeks there is a ridge of 

 thick epithelium covering the incisor part of the jaws, but, as 

 in older specimens, entirely confined to that part. A similar 

 condition seems to exist in all the higher mammals, though 

 in some the proliferated area extends to the premolar region. 

 In Marsupials the thick layer of epithelium, besides covering 

 the incisor part of the jaws, passes back to the molar region, 

 and differs from the typical mammalian condition in extending 

 to and covering the inner part of the lips. 



That the thickened epithelium on the sides of the mouth in 

 the marsupial is not specially developed to protect the jaws 

 while grasping the nipple may be inferred from the fact that 

 the tongue and the palate, which have almost the entire share 

 in holding the nipple, are coated with a uniform compara- 

 tively thin layer of squamous epithelium. Even in placental 

 mammals the epithelial armature of the jaw is developed at 

 such an early date as to suggest its being the remains of an 

 ancestral horny beak. 



Geological strata have not yet yielded any evidence of the 



