DIGESTIVE SYSTEM 



731 



Epithelium. 



Fig. 429. 



I, II, III, IV, Diagrams of developing 

 tooth (After Hill). 



V, Section through the skin of an Elas- 



upper lip do not meet, a harelip and 

 cleft palate result. 



In some birds the two halves 

 of the palate never unite. In some 

 mammals, such as the cat and dog, 

 the two portions forming the upper 

 lip have not united as well as they 

 have in the human being, and, con- 

 sequently, a deep median groove 

 called the philtrum, remains. This 

 line can be seen to run along the 

 entire septum of the nose extern- 

 ally. 



Next in order of study come the 

 teeth, tongue, tonsils, glands of the 

 mouth cavity, and glands of the 

 pharyngeal pockets. 



There are two types of teeth 

 which have no relationship to each 

 other in their origin. The true 

 teeth are akin to placoid scales 

 (Fig. 429). They arise by a calcar- 

 eous secretion at the junction where 

 ectoderm and mesenchyme meet 

 and are thus a product of both 

 layers 



The other type comes purely 

 and simply from the cuticle and is 

 formed by what is known as cornifi- 

 cation or hardening of the epithe- 

 lium (Fig. 422, 6). The parts which 

 have invaginated to form the stoma- 

 todeum retain the capacity to form 

 hard structures, consequently, any 

 portion of the mouth-walls may se- 

 crete scale substances. It is neces- 

 sary to appreciate this in order to 

 understand that in the different 

 types of fish and amphibia, teeth of 

 almost any number, size, and shape 



mobranch showing formation of a dermal 

 spine. Highly magnified. 



1. Horny layer of ectoderm. 2. Malpig- 

 hian layer. 3. Columnar cells of ectoderm 

 secreting. 4. Enamel. 5. Dentine (black). 

 6. Dentinal pulp. 7. Connective tissue. 

 (From Shipley and MacBride). 



