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 fi/tk arches are rudimentary 



,ween the maxillary processes and the mandibular arch the buccal cavity or 

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



Processus globularis. 



Mouth of olfactory 

 pit, or nostril. 



Palatal process of pro- 

 cessus globularis. 



Palatal part of maxil- 

 lary process. 



Maxillary 

 process. 



/Lens. 



Mouth 

 cavity. 



FIG. 717. The roof of the mouth of a human embrvo of about two and a half mnnth* 

 mode of formation of the palate. (His.) (From Marshall's Ko^r^^. old, showing the 



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



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



thus enclosed within the body of the embryo and 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 mesoblast 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 pharyngeal septum 



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



tion is established between the mouth and the future 



pharynx. The oral pit or stomodseum 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 stomodseum a pocket-like involution of the epiblast, 

 the pouch of Rathlte, extends upward between the trabeculae cranii toward the 

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

 dseum, 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 prcecervicalis, 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.) 



