254 



STUDY OF PIG EMBRYOS. 



tinguished 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. 147 and 148, arach), easily distinguishable even with a low power by its 

 light coloration. It consists of widely separated cells connected together by 

 very distinct processes, and is permeated by a number of small blood-vessels 

 running in various directions through the layer. Its external boundary is now 

 very distinct, being marked by a layer of somewhat crowded, elongated cells 

 which merge on the side toward the ectoderm into the general surrounding mesen- 



nas.tb. 



Max. 



\ — niax.tb. 



j AIx.sup. 



Mdh. 



Fig. 146. — Pig, 20.0 mm. Frontal Section of Head. Series 40, Section 68. 

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

 MM, Mandible. Mx.sup, Superior maxillary nerve. nas.tb, Naso-turbinal fold. Sept, nasal septum. 

 Sk, Mesenchymal anlage of the dura mater and skull. X ^^ diams. 



chyma. Out of this denser layer (Figs, 146 and 147, Sk) arise both dura mater 

 and the membrane bones of the skull. 



Section through the Anterior Part of the Snout (Fig. 146). — On the dorsal side 

 appear the two 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 prolongation 



