FRONTAL SECTIONS OF THE HEAD, EMBRYO OF 24 MM. 259 



struction is made these identifications can be recorded as tentative only. The 

 pharynx, Ph, appears as a yolk-shaped slit lined throughout by entoderm. From 

 its median ventral floor rises the great mass of the tongue, Ton, over which the 

 dorsal roof of the pharynx forms a closely fitting arch. A portion of the epi- 

 thelium of the tongue is loosened from the underlying tissue, probably owing to 

 defective preservation. Upon the lower side of the tongue extend downward the 

 anlages of the hyoglossal muscles, hy. gl, between which are situated the lingual 

 arteries, art. On either side, in the part of the section corresponding to the man- 

 dible, appears Meckel's cartilage, Mk, a somewhat conspicuous and easily identi- 

 fied structure, owing to its dark staining. Meckel's cartilage is the primitive 

 skeletal element of the mandibular arch, and is homologous with the cartilagin- 

 ous jaw of the lower fishes. It is, for the greater part, a purely embryonic struc- 

 ture, the mandible of the adult being a secondary bone. By referring to figure 

 99 (pig 10 mm.), it can be seen that the mandibular arch extends upward to- 

 ward 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 rod-like structure ex- 

 tending the entire length of the arch. Its upper end is, therefore, 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 mallev^. A little outside of Meckel's cartilage in our section 

 is the inferior maxillary nerve, Mx. i, and still further lateralwards is a small, 

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



Pig Embryo of 24 mm. 



Section through the Eye. — 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 retina and 

 pigment layer. The origin of the retina and lens is illustrated by the chicken 

 embryo (Figs. 155, 156), and in a more advanced stage by the pig of 12 mm. (Fig. 

 116). There is added here figure 1 49 , from a section of the eye of a rabbit embryo 

 of thirteen days, in order to facilitate the comparison between the 12 mm. stage 

 and the 24 mm. stage of the pig. In figure 149 the ectoderm, Ec, forms an arch 

 over the eye and indicates the commencing formation of the cornea, the layer of 

 ectoderm being destined to become the external epithelium of the cornea. Be- 

 tween the lens and the retina there has been an ingrowth of tissue accompanied 

 by blood-vessels, which form a more or less distinct covering over the surface of 

 the lens and constitute the so-called tunica vasculosa, tu. v. The space between 



