190 STUDY OF PIG EMBRYOS. 



surrounding mesenchyma. 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 dorsal wards, a second nerve. Both of these are 

 branches of the glosso-pharyngeus, and lie behind the cleft. They are, there- 

 fore, termed the post-trematic branches. Below the cleft and somewhat on its 

 median side is a similar third nerve-branch, the pretrematic of the glosso- 

 pharyngeus, running in front of the cleft. The outline of the embryo forms a 

 rounded eminence outside of the second cleft ; it represents in part the hyoid arch. 

 In the midst of the mesoderm of this appears a light area with a few nerve-fibers, 

 the end of the facial nerve, N. 7. 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 the mandibular process there are light spaces differing in their exact distribu- 

 tion on the two sides of the specimen. These spaces contain nerve-fibers and 

 they represent the inferior maxillary nerve. We now come to the oral fissure, 

 O. F, which separates the body from the head. In the head portion of the sec- 

 tion we have the maxillary process, MX, which is separated in part from the rest 

 of the head by the deep lachrymal groove, L. gr. On either side there shows a 

 shaving from the epithelium of the olfactory chamber, Olf. The fore-brain has 

 expanded laterally, L. V, to form the lateral ventricles, the walls of which, H, are 

 the anlages of the cerebral hemispheres. On the dorsal side, which is the lower 

 side in the figure, the hemispheres project somewhat, leaving a median space be- 

 tween them. This median space is filled with mesenchyma, which may already 

 be regarded as the anlage of the falx. In the tissue of the falx are two very 

 small blood-vessels, the forward prolongations of the lateral jugulars, which are 

 to unite to form the median superior longitudinal sinus. In the previous section 

 these vessels also reappear, but are already united (Fig. 116). In the median 

 dorsal line the wall of the fore-brain is thin and shows a characteristic notch. 

 Close to the surface of the fore-brain there is a very distinctly marked vascular 

 layer, the commencing pia mater, P. M, and with a high power it can be easily 

 seen that the differentiation of the arachnoid zone has already begun. 



Section through the Third Gill Cleft and Nasal Pits. In this section the head 

 is clearly separated by a considerable space from the rest of the section. The 

 transverse diameter of the embryo is here much less than higher or lower, 

 so that the section as a whole seems somewhat narrow. It shows the entire 

 length of the third gill cleft, cl. Ill, exhibiting, on one hand, its connection with 



