18 



MORPHOGENESIS 



Uned with endoderm. The first internal groove becomes transformed into the auditory (Eu- 

 stachian) tube, tympanic cavity, etc. The second internal groove persists in part as the fossa of 

 the vakUine tonsil The third and fourth grooves are probably represented m part by the 

 vallecula and recessus pinformis, detached portions of their hning endoderm giving rise to the 

 thymus, parathyreoid glands, etc. The rudimentary fifth groove is said to give rise to the 

 ultimobranchial body, a structure of uncertain sigmficance (fag. 27). :„„i„j,, ^u^ 



Development of the face.— The facial region is at first relatively smaU. It includes the 

 sense organs (eye, ear, nose) and mouth region Some of the more important devebpmental 

 features may be briefly mentioned. In an embryo of the sixth week (fig 28) the wide mouth 

 aperture is seen to be bounded below (posteriorly) by the lower (jnandibular) portion of the 



Fig 22 —Human Embryo of 4.2 mm., Showing Three Branchial Grooves. 



(After His.) 



Auditory vesicle 



Branchial grooves 



Upper limb 



Mesodermic somite 



Lower limb 



/ 



firet'arch, laterally by the upper (maxillary) process of the first arch. Above it is boundedrby a 

 median plate, the nnsal process, which on either side forms a protuberance, the globular process. 

 Lateral to the nlobular process is a rouiidcd (U^iJiession, the nasal pit. The maxillary process 

 extends forward and fuses with the globular process to form the upper jaw region (failure to 

 unite resullinc in the malformation known as 'hare-lip'). The nose is at first broad, due to 

 the width of tlio nasal process, v/h'u;h later becomes the nasal septum (fig. 29). The nasal pits 

 deepen and later acquire openings into the primitive mouth cavity. 



The viscera. — The structures so far considered belong, for the most part, to 

 the body wall; it remains to consider the general plan of arrangement of the 

 viscera. It has been pointefl out that the body may be regarded as a cylinder 

 enclosing two tubes, one of which constitutes the central nervous system and the 



