330 



STUDY OF PIG EMBRYOS. 



a purely embryonic structure, the mandible of the adult being a secondary bone. 

 By referring to figure 166 (pig, 10 mm.), it can be seen that the mandibular arch 

 extends upward toward the otocyst and forms the boundary of the first gill-cleft, 

 the outer division of which becomes the meatus auditorius externus. In other 

 words, the upper portion of the mandibular arch is in close proximity to the 

 otocyst and the anlage of the tympanum or middle ear. Meckel's cartilage is a 



R 



. N 



FIG. 222. RABBIT EMBRYO OF THIRTEEN DAYS; SECTION OF THE EYE. ; 



EC, Epidermis. L, Lens, mes, Mesenchyma. N, Anlage of optic nerve. P, Pigment layer. R, Retina, tu.v , 



Tunica vasculosa lentis. 



rod-like structure extending the entire length of the arch. Its upper end is, there- 

 fore, close to the future tympanum. While the greater part of Meckel's cartilage 

 disappears during later development, the upper end persists and takes a direct 

 share in the formation of the malleus. A little outside of Meckel's cartilage in 

 our 'section is the inferior maxillary nerve, Mx.i, and still farther lateralward is a 

 small, darkly stained body, x, which has not yet been identified with certainty. 



Pig Embryo of 24 mm. Study of Sections. 



Section through the Eye (Fig. 223). In the pig of 24 mm. the anlages of all 



the parts of the adult eye may be said to be present, with the exception of the 



pigment layer of the iris, which arises somewhat later by a forward growth of the 



