16 THE DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



and with this manner of drainage, as illustrated in figures 22 and 23, one must feel 

 that the provision for the blood-circulation is well designed and is perfectly adequate 

 for the structures as then existing, and were there no further alterations in the 

 structures themselves no further change in their blood-vessels would be necessary. 

 On the other hand, there are no superfluous channels present and no evident vascular 

 provision for structures not yet developed, that is, disregarding the potential pro- 

 liferative power which exists in such an endothelial system. The establishment of 

 this primary type of the circulation of the head completes our second period in 

 the growth of the cranial vascular system. 



CLEAVAGE OF BLOOD-VESSELS OF HEAD INTO SEPARATE SYSTEMS. 



The differentiation of the dura mater and the formation of the arachnoid mesh 

 begins in the region of the base of the skull, and from there the process spreads 

 slowly upward toward the vertex of the head. As these structures form it can be 

 seen that the anastomosing channels connecting the capillary sheet of the brain 

 with the more suj^erficial drainage channels are gradually closed off, the more 

 ventral ones first and the more dorsal ones later. In this way the dura forms a 

 partition that results in a general separation or cleavage of the superficial vessels 

 (consisting mainly of the primary head-vein and its tributaries) from the deeper 

 vessels in intimate contact with the brain-wall, including the capillary sheet and 

 the vessels supplying and draining it. The latter or fleeper system, however, 

 continues to drain into the former at certain restricted places. As this cleavage 

 occurs, we can distinguish between dural vessels which are chiefly veins and cerebral 

 or pial vessels which include arteries as well as veins. Soon after, coincident with 

 the formation of the membranous skull, the dural system becomes more or less 

 completely separated from the vessels of the integument and its subjacent soft 

 parts. We then have three different main strata of blood-vessels — the external, 

 the dural, and the cerebral. The most conspicuous of the early external vessels 

 are those belonging to the integument. They make their appearance around the 

 base of the skull in embryos between 12 mm. and 20 mm. long and spread ujiward 

 toward the vault. In spreading upward they exhibit a characteristic growing 

 edge consisting of anastomosing loops of the mesh which can be seen with the 

 naked eye as an advancing narrow line marking off the non-vascularized area 

 above from the vascularized part below, as has been clearly pointed out by Hoch- 

 stetter (1916) and as indicated in our figure 10. 



The first steps in this cleavage are well under way in embryos 14 mm. long. 

 An embryo of this stage (Carnegie Collection, No. 940) is shown in figure 1. 

 The veins of the head were distended with a natural blood injection and at the 

 same time the surrounding tissues were quite transparent; as a result, it was pos- 

 sible to determine their arrangement from a surface examination with consid- 

 erable detail. A photograph was made of the specimen, and in it were added 

 the details that could be seen with the aid of a binocular microscope. It was in 

 this way that figure 1 was obtained. What is seen there is the primary head-vein 

 and its tributaries. To see the deeper structures it was found necessary to make a 



