THE EMBRYO. 



1171 



great cornu of the hyoid bone, while the body of this bone is formed between the 

 second and third arches. The fourth and fifth arches are rudimentary. 



Between the maxillary processes and the mandibular arch the buccal cavity or 

 mouth is formed. As has already been stated (page 1157) the cephalic end of the 



Mouth of olfactory 

 pit, or nostril. 



Palatal process of pro 

 cessus globularis. 



Palatal part of maxil- 

 lary process. 



Maxillary 

 process 



Mouth 

 cavity. 



FIG. 717. The roof of the mouth of a human embryo of about two and a half months old showine the 

 mode of formation of the palate. (His.) (From Marshall's Vertebrate Embryology.) 



embryo becomes remarkably curved on itself, the fore-brain and mid-brain bendino- 

 downward over the anterior portion of the original blastodermic vesicle, which is 

 thus enclosed within the body of the embryo arid con- 

 stitutes the fore-gut ; the fore-gut terminates in a blind 

 extremity beneath the head (Figs. 720 and 759). 

 Another prominence, the rudimentary heart, appears on 

 the ventral surface of the fore-gut. Between these two 

 prominences, caused by the projection of the fore-brain 

 and the heart, an involution of the epiblast takes 

 place, gradually deepening until it comes in contact 

 with the blind end of the fore-gut. This is the oral 

 pit or stomodceum, already referred to ; it presents the 

 form of a pentangular opening, bounded in front by 

 the fronto-nasal process, behind by the mandibular 

 arch, and laterally by the maxillary processes. From 

 the beginning the meso blast is absent in the region of 

 the oral pit, and hence its epiblastic lining meets the 

 hypoblastic covering of the blind anterior end of the 

 fore-gut and forms a thin septum, the phari/ngeal septum 

 (Fig. 759) : this soon breaks down, and a communica- 

 tion is established between the mouth and the future 

 pharynx. The oral pit or stomodaeum is not equivalent 

 in extent to the adult mouth, since the latter includes the tongue, which is devel- 

 oped from the floor of the pharynx ; in fact, as His has pointed out, the ante- 

 rior pillars of the fauces are developed from the second branchial or hyoid arch. 



From the upper part of the stomodaeum a pocket-like involution of the epiblast, 

 the pouch of Rathke, extends upward between the trabeculse crariii toward the 

 thalamencephalon. This involution ultimately loses its connection with the stomo- 

 d;eum, and, becoming applied to the infundibulum, forms the anterior lobe of the 

 pituitary body (Fig. 720). 



The anterior visceral arches grow more rapidly than the posterior, with the 

 result that the latter become telescoped within the former, and a deep depression, 

 the sinus prcecerviealis, is produced. This sinus is bounded in front by the hyoid 

 arch, and ultimately becomes obliterated by the fusion of its anterior and posterior 

 walls. 



FIG. .718. Head of a human em- 

 bryo of about eight weeks, in which 

 the nose and mouth are formed. 

 (His.) 



