THE ORGANS OF THE INNER GERM-LAYER. 



305 



NT- 



although free from papillae, is richly provided with follicular glands. 

 It is developed out of two ridges in the region where the second and 

 third visceral arches come together in the median plane. The 

 anterior and posterior fundaments unite in a V-shaped furrow, 

 the arms of which diverge in front. The circumvallate papillae are 

 formed on the body of the tongue along this furrow, which persists 

 for a long time. Where the two arms of the V meet there is a deep 

 pit, the foramen co3cum, which His has brought into connection with 

 the origin of the thyroid glands, which will soon be discussed. 



(2) The Salivary Glands are demonstrable even in the second 

 month. The 'fundament of the submaxillary appears first in human 

 embryos at the sixth week 



(CHIEVITZ), afterwards the 

 parotid in the eighth week, 

 and finally the sublingual. 



(3) From a morphological 

 point of view, the Teeth can 

 well be designated as the most 

 interesting structures of the 

 oral cavity. Their develop- 

 ment in Man and Mammals 

 is accomplished in a manner 

 which is neither simple nor 

 easily intelligible ; in the 

 lower Vertebrates, on the con- 

 trary, it is simpler, and for that reason I shall make use of the latter 

 as the starting-point of the description. 



The teeth, which in Mammals are attached to the edges of the jaws 

 and only bound the entrance to the alimentary tube, possess in the 

 lower Vertebrates a very wide distribution. For in many species they 

 not only cover the roof and the floor of the oral cavity and the inner 

 surface of the branchial arches in immense numbers, as palatal r 

 lingual, and pharyngeal teeth, but they are also distributed in close-set 

 rows over the whole surface of the skin, and produce, as in the 

 Selachians, a strong and at the same time flexible coat of mail. 



The teeth are originally nothing else than ossified papillae of the skin 

 and the mucous membrane, upon the contiguous surfaces of which they 

 are formed. The development of the dermal teeth in Selachians shows 

 this in a very convincing manner. 



In young Shark embryos, by a proliferation on the part of the sub- 

 epithelial cells, there are developed on the otherwise smooth surface 



20 



Fig. 168. Tongue of a human embryo about 20 

 mm. long, neck measurement. After His, 

 "Menschliche Embryonen." 



