180 STUDY OF PIG EMBRYOS. 



Jug'" , of the jugular nearest the fore-brain lie the very small sections of the 

 fourth, A../, and third, N.j, cerebral nerves. The fourth nerve is minute in size 

 and lies just behind the jugular. The third nerve, though somewhat larger, is 

 also very small and lies anterior to the jugular somewhat on its medial side. Both 

 of these nerves, owing to their small dimensions, are somewhat difficult to ob- 

 serve with the low power. The detailed figure brings out more clearly other 

 points. It shows very clearly the junction of the trigeminal, N. j, and acustico- 

 facial, N. 7, S, ganglia with the wall of the hind-brain, and also the division of 

 that wall into its three primary layers, the ectoglia, Ec. gl, the gray layer, tin, and 

 the inner or ependymal layer, Epen, and also the median floor-plate or raphe, 

 Raph. Immediately below it is the basilar artery. On either side of the fore- 

 brain is the section of the loop of the carotid, A. car, which is passing up to join 

 the anterior end of the basilar artery, which last has been produced by the fusion 

 of the two originally symmetrical vertebral arteries. This portion of the carotid 

 loop probably corresponds to the vessel designated in the adult as the posterior 

 communicating branch, by which the end of the carotid proper anastomoses with 

 the basilar artery. At the side of the fore-brain appears a blood-vessel, Jug. L, 

 which might be called the lateral jugular. It is a branch of the main jugular 

 stem and passes over the side of the fore-brain toward the median dorsal surface 

 thereof, where it meets the corresponding vein of the opposite side, with which it 

 then unites to form a single median vessel. This vessel ultimately acquires 

 great size, and is known as the superior longitudinal sinus. It is shown in figure 

 105. So much of the vessels as do not unite in the median line persist, to form the 

 lateral sinus of the adult. These sinuses in the embryo are all small branches of 

 the veins when they first appear. Their great enlargement does not occur until 

 comparatively advanced stages. Finally, attention should be paid to the follow- 

 ing important modifications in the mesenchyma. Already there has been a rich 

 development of a plexus of fine blood-vessels over the surface of both the fore- 

 and hind-brain which has been accompanied by a slight condensation of the mes- 

 enchyma between the blood-vessels, thus marking a distinct membrane, in which 

 we can easily recognize the pia mater, Pia. Outside of the pia mater comes a 

 relatively broad zone, Arach, in which the cells are widely separated from one 

 another and are connected by slender and long processes so that the intercellular 

 spaces are very extensive. This broad zone is the anlage of the arachnoid mem- 

 brane. It is much more differentiated around the ventral portion of the brain 

 than around the dorsal side. Between the arachnoid zone and the external 

 epidermis the mesenchyma is somewhat more condensed and the cells are elon- 

 gated in form, in part almost spindle-shaped, forming a layer, Cut, which we may 

 consider the anlage of the cutis, and perhaps, also, of the subcutaneous tissue. 



