164 GEORGE L. STREETER 



the blood vessels. The profile reconstruction shown in figure 6 is 

 based on series No. 96 and was made by preparing tracings on 

 transparent paper which were then superimposed and a composite 

 tracing made of the whole series. This is about the same stage 

 that is shown by Markowski in his figure 4. 



By this time the venous drainage of the cranium is estabhshed 

 along channels that correspond fairly well to those found in 

 the adult. It is clearly subdivided into three separate systems: 



(1) The superficial system draining the integument and soft parts ; 



(2) the dural system lying between the dura and bone; and (3) the 

 cerebral system. All three are originally outgrowths of the same 

 capillary plexus. The separation of the dural veins and the 

 cerebral veins we have traced through, step by step. The super- 

 ficial veins in embryos 20 mm. long are already separated off from 

 the dural system by the membranous and cartilaginous cranium. 

 They appear first in the lower parts of the head, where they were 

 originally separated off from the deep veins, and form a plexus 

 that gradually spreads upward over the vault. They maintain a 

 few anastomoses with the dural system, which constitute the so- 

 called emissary veins; one of these is shown in figure 6. Aside 

 from the channel maintained through the orbit the chief drainage 

 from the superficial system is through the external jugular vein, 

 which is pictured by Salzer ('95) as already present in guinea-pig 

 embryos 20 mm. long. 



On examining the dura in embryos 50 mm. long it will be seen 

 that for the greater part it closely invests the interior of the 

 developing cranium and is relatively poor in blood vessels. This 

 is true especially in those portions where the cartilaginous and 

 bony cranium is more advanced in its differentiation, as in the 

 base of the skull and in the frontal, temporal and lower occipital 

 regions. In other regions the dura projects within the cranial 

 cavity, being separated from the future bony skull by a layer of 

 areolar tissue, in the meshes of which are found the large blood 

 channels and their tributaries. The largest area of this kind is 

 found situated over the mid-brain and extending from the caudal 

 margin of the cerebral hemispheres to the cerebellum. This area 

 extends laterally down to the base of the skull, narrowing as it 



