348 THE ANIMAL KINGDOM. 



the oral cavity. This cavity becomes divided in Chelonia, Crocodilia, some 

 Aves and all Mammalia into an upper nasal portion, and a lower buccal 

 portion by the formation of a hard or, in Mammals, of a hard and soft 

 palate. Teeth and glands are developed in the mouth. The teeth are 

 composed typically of two substances, enamel and dentine. The former is 

 derived from the inner surface of a cellular structure, the enamel organ, 

 which is formed by a downgrowth of the epiblastic rete mucosum, covering 

 an upgrowth of the mesoblast, the dental papilla. The cells on the outer 

 surface of the latter or the odontoblasts give origin to the dentine, whilst 

 the central part of the papilla persists as a vascular pulp supplied also with 

 nerves. The teeth formed in this manner are phylogenetically identical 

 with the exoskeletal spines of certain Fishes (Elasmobranchit). A third 

 substance, cement (crusta petrosa), formed by the connective tissue sur- 

 rounding the base of the tooth, agrees histologically with bone, but is found 

 only on the roots of some teeth when implanted in sockets. The teeth are 

 attached to the underlying jaws either by a fibrous membrane (some Fish) 

 or by bone, i.e. by anchylosis (some Fish, Amphibia and Reptilia), or are 

 implanted in grooves or sockets (a few Fish, Reptilia and all Mammalia). 

 Their shape, situation, number, &c., vary immensely. They are found in 

 the pharynx of many Fish, and here the enamel organ is derived from 

 hypoblast and not epiblast as in the stomodaeal region. New teeth in suc- 

 cession to old teeth are either formed without limit of numbers, as in most 

 Pisces, Amphibia, Reptilia, or are restricted to a second set in some Mam- 

 malia. In other Mammalia there are no successional teeth at all. The 

 enamel organ of these successional teeth is typically derived from the pedicle 

 of a pre-existing enamel organ, but in Teleostei independently from the rete 

 mucosum. A partial absorption of the old tooth generally occurs when it 

 is replaced. In some Mammalia the teeth grow from persistent pulps as 

 fast as their exposed surfaces are worn away, e. g. the incisors of Rodentia. 

 Specialised oral glands are found from Amphibia onwards. Their structure, 

 number and position vary much, but Mammalia are characterised by three 

 well-developed pairs parotid, sub-maxillary and sublingual, in addition to 

 minor glands. A muscular growth from the floor of the mouth above the 

 basihyal region constitutes the tongue. It is well developed in Amphibia 

 and higher forms. 



The mesenteron (archenteron) is divisible into two or three regions. 

 The first, fore-gut, extends from the stomodaeum to the point of origin of 

 the liver, and comprises a pharynx, oesophagus, and stomach. The second 

 constitutes the mid-gut or small intestine ; the third the hind-gut, large in- 

 testine, or colon. The two latter are sometimes difficult to distinguish. 

 Their limits are either indicated by the presence of one or two lateral caeca, 

 by a difference of calibre or of the mucous membrane. A post-anal section 

 of the mesenteron has also been distinguished in the embryo, but some doubt 



