CHAP. X.] THE DEVELOPMENT OF THE CAT. 343 



external surface of the pre-axial end of the embryo, tlie mouth 

 appears as a depression on the surface of the front end of the head. 

 This depression is bounded above by the naso-frontal process and 

 below by the first visceral arch, and the tongue arises in the floor of 

 the chamber thus formed. This primitive buccal cavity extends 

 back beneath the cerebral vesicles till it meets the blind end of the 

 front of the pharynx, into which it soon opens by a process of 

 absorption of the intervening partition formed by the just mentioned 

 lamina ventralis, part of which partition persists as the velum 

 palati. An upgrowth from the buccal cavity, the pituitary body, 

 meets a downgvowth (between the trabccuLnc) of the brain — the 

 infundibulum. It is thus these parts are formed. Afterwards the 

 pituitary body becomes cut off from the buccal cavity and adheres 

 to the infundibulum. 



The amis is formed in an analogous manner at the post-axial 

 end of the embryo. Here at first the intestinal groove is continuous 

 with the medullary groove round the hind end of the chorda 

 dorsalis, but the ventral lamina descends behind, as a single plate 

 not divided into an outer somatopleure and an inner splanchnopleure, 

 and closes in the hinder end of the alimentary tube. The hinder- 

 most part of this primitive tube is called the cloaca. Beneath the 

 posterior end of the chorda a depression of the epiblast appears ; 

 this deepens, and the mesoblast of the ventral lamina is absorbed, 

 a perforation being effected and the rectum and anus thus formed. It 

 is from the ventral wall of the cloaca that the allantoic sac before 

 described grows out, but it grows out beyond the point where the 

 ventral lamina begins to be differentiated into somatopleure and 

 splanchnopleure. It grows out therefore from the latter into the 

 interspace dividing those lameUar subdivisions of the mesoblast. 



The teeth are developed each by a double process. In the place 

 for each tooth that is to be, a depression takes place in the mucous 

 membrane of the gum, the deepest layer of the epithelium thicken- 

 ing and extending into such depression. The sac of epithelium 

 thus formed widens at its deepest part, while simultaneously a papilla 

 of the mucous membrane rises up from below and pushes itself into 

 the deep surface of the widened part of such epithelial sac. The 

 mucous papilla ultimately becomes the mass of the future tooth, 

 while the deepest surface of the epithelial inflection (into which the 

 ascending papilla has protruded) becomes the enamel of the tooth, 

 on which account it is called the " enamel organs Thus each tooth 

 arises from two distinct sources. In the process described, the neck 

 of the epithelial depression becomes exceedingly attenuated, while 

 what is called a " dental sac " forms itself about the developing tooth 

 by direct transformation of the mucous membrane, into an outer 

 layer of fibrous tissue with a vascular layer within. The contained 

 papilla then assumes the shape of the crown of the future tooth, 

 and soon begins to calcify, thin caps of dentine being formed^ on 

 the milk teeth within the gum, at an early period during gestation, 

 each cap thickening by degrees as it extends its margins. As soon 



