274- STUDY OF PIG EMBRYOS. . 



figure. On the ventral side of the vein there appears a darkly stained mass, 

 N.io, the nodosal ganglion of the vagus nerve, and outside of this ganglion is 

 the section of the spinal accessory nerve. Immediately below the nodosal ganglion 

 we have the internal carotid artery, car.in. A little to the inside of the jugular 

 is a small vessel, Ao.D, of great morphological importance. The corresponding 

 vessel appears on the opposite side. Although, very small, this vessel has a dis- 

 tinct coat of condensed mesenchyma around its endothelium. The two vessels 

 are the descending aorta, which have almost completely aborted, and in slightly 

 older specimens will be found to have disappeared altogether. The descending 

 aortae are the longitudinal trunks by which the dorsal ends of the five aortic 

 arches of early stages are connected together. The portion shown in this section 

 is the part of the descending aorta between the tops of the third and fourth aortic 

 arches. The relations are shown in the reconstruction (Fig. 172). The pharynx, 

 Ph, is narrow in its dorsal ventral diameter, but wide transversely, and offers the 

 very characteristic yoke-shaped figure in the section. The distal portions of the 

 second gill-clefts are shown, and they .appear disconnected from the pharynx, 

 the connection occurring in sections higher up. Each cleft is somewhat slit-like, 

 so that its cavity is an oblique fissure and somewhat parallel in position to the 

 first cleft (Fig. 192). Both the pharynx and the gill-clefts are, of course, lined 

 throughout by entoderm, which forms a sharply defined layer crowded everywhere 

 with nuclei, which are of about the same size as those of the surrounding mesen- 

 chyma. In the pharynx the entoderm is somewhat thinner on the dorsal than 

 on the ventral side. In the clefts it is thicker than in the pharynx proper, and 

 especially in the clefts it may be observed that the mitotic figures always occupy 

 a superficial position. On the dorsal side of the cleft is a very small blood-vessel, 

 near which, with a higher power, one may see a small nerve, and nearby, but 

 more dor-salward, a second nerve. Both of these are branches of the glosso- 

 pharyngeus, and lie behind the cleft. They are, therefore, termed the post-trematic 

 branches. Below the cleft and somewhat on its median side is a similar third 

 nerve-branch, the pre-trematic of the glosso-pharyngeus, running in front of the 

 cleft. The outline of the embryo forms a rounded eminence outsidf^of the second 

 cleft; it represents in part the hyoid arch. In the midst of .the mesodenii ".$ this 

 appears a light area with a few nerve-fibers, the end of the facial nerve, A . , 

 The mandibular arch or process, Mdb, is very distinct and prominent. It is 

 separated from the hyoid arch by a deep external notch, which corresponds to 

 the external first or auditory cleft. In the interior of trie mandibular process 

 therc t are light spaces differing in their exact distribution on the two sides of 'the 

 These spaces contain n live-fibers and they represent the inferior maxil- 

 . We now come to the oral fissure,/0.jF, which separates the body * 



he head. In the head portion of the section we have the maxillary process* 

 hich is separated in part from the rest of the head by the deep lachrymal 

 L.gr. On either side there/shows a shaving from the epithelium of tse ' 



