FRONTAL SECTIONS OF HEAD, EMBRYO OF 20 MM. 



325 



around the brain is clearly demonstrated. The pia mater is very distinct. In those 

 parts of the sections where the brain-wall is cut obliquely, it can be distinguished 

 only by a somewhat careful observation, as the tissues of the pia mater and of 

 the brain overlap. All about the brain is the broad zone of the arachnoid (Figs. 

 220 and 221, arach), easily distinguishable even with a low power by its light colora- 

 tion. It consists of widely separated cells connected together by very distinct pro- 

 cesses, and is permeated by a number of small blood-vessels running in various direc- 

 tions through the layer. Its external boundary is now very distinct, being marked 

 by a layer, Sk, of somewhat crowded, elongated cells which merge on the side 



Sept. 



Jk.o. - 



Max. 



nas.tb. 



nax.tb. 



MX. sup. 



Mdb. 



FIG. 219. PIG, 20.0 MM. FRONTAL SECTION or HEAD. SERIES 40, SECTION 68. 



H, Cerebral hemisphere. Jk.o, Jakobson's organ. Max, Maxillary process, max.tb, Maxillo-turbinal fold. 

 Mdb, Mandible. MX. sup, Superior maxillary nerve, nas.tb, Naso-turbinal fold. Sept, Nasal septum. Sk, 

 Mesenchymal anlage of the dura mater and skull. X 18 diams. 



toward the ectoderm into the general surrounding mesenchyma. Out of this 

 denser layer (Figs. 219 and 220, Sk) arise both dura mater and the membrane 

 bones of the skull. 



Section through the Anterior Part of the Snout (Fig. 219). On the dorsal side 

 appear the two cerebral hemispheres, H, cut separately and each showing the 

 cavity of its lateral ventricle. On the ventral side the mandible, Mdb, is cut 

 separately and is separated by the oral fissure from the rest of the section. The 

 maxillary processes, Max, are large, and each is furnished with an inward prolonga- 

 tion extending toward the median line. From the oral fissure there extend upward 



