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very much with the sharpness of the arterioles and making it impossible 

 to define the veins, or embrjronic sinuses. So constant is this extravasa- 

 tion in position and degree that it often seems as if the arachnoid spaces 

 communicate freely with the veins, but, as will be shown presently, this 

 is not the case. 



In the smallest specimen (No. 225, 46 mm. long) the middle cerebral 

 artery and the arteries to the mid-brain are well injected, but in no case 

 does the injection extend into the brain substance. The arachnoid spaces 

 are filled evenly with the blue injecting fluid, but there is none within 

 the ventricles. Since the fluid does not reach the capillaries, it is evident 

 that the extravasation took place from the arterioles, and this seems to be 

 the case, for the arterioles are easily torn at the point they enter the brain 

 substance. In the early stages the brain is attached only slightly to the 

 embryonic pia mater, and it is practically impossible to remove the 

 brain with its pia mater intact, as can be done in older embryos or in 

 the adult. At the point the vessels leave the pia mater to enter the brain 

 substance the blood-vessels have but a single endothelial wall, and it is 

 here that the rupture and extravasation take place when these arteries 

 are injected. 



In an embryo a little older, 'No. 237, Fig. 1, the injection of 

 the artery is practically perfect, and I have therefore given a drawing 

 of it. The brain was peeled out with its pia mater only with difficulty 

 and over the region of the lateral cerebral fissure (Sylvius) some of the 

 vessels separated and remained attached to the dura. This portion was 

 drawn inverted and redrawn upon the brain, and the point at which the 

 main trunks are torn off is indicated in the drawing in the region of the 

 island. The injection is practically a complete arterial injection with 

 but little extravasation into the arachnoid and none into the ventricles. 

 An extravasation is over the region of the island, on both sides, and to 

 a slight extent over the mid-brain on one side. The arteries divide and 

 subdivide in regular fashion until the terminal branches are reached, 

 when they turn at right angles to enter the brain substance. There are 

 from five to ten of these cortical arteries to each square millimeter of 

 brain surface. Around some of them there is some extravasation of 

 Prussian blue, indicating the way the blue enters the arachnoid spaces. 



Over the surface of the brain of an embryo 65 mm. long (No. 234^) 

 there are numerous blue spots, about one to each square millimeter. 

 Where the spots are larger there is a tendency for them to run together, 

 but in general the brain is only spotted rather than being covered evenly 

 with an extravasation. There is no extravasation in the ventricle. In 

 another brain of about the same age (No. 235, 59 mm. long) the ex- 



