DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 81 



connects with the aorta by chains of angioblasts which are forerunners of direct 

 dorsal branches of the aorta. 



Exactly when the angioblasts along the fort-brain and the midbrain can be 

 identified has not been determined, but at the stage of 12 somites there is a plexus 

 of large vessels along the lateral surface of the forebrain and midbrain extending 

 to the ventral surface of the forebrain at the base of the optic vesicle and anas- 

 tomosing with the primary vascular channel of the hindbrain. This plexus 

 connects with the aorta just at the base of the optic vesicle, as was shown by 

 Evans in his figure 398 for a duck embryo of 13 somites in the " Manual of Human 

 Embryology" (Keibel and Mall). At the stage of 12 somites this plexus also 

 connects with the aorta opposite the midbrain, as shown in text-figure 1. 



This deep primary plexus, which I have uniformly represented in a gray 

 tone, soon gives rise to a superficial plexus opposite the region of the forebrain 

 and the midbrain, as shown in text-figure 1. In this superficial plexus there 

 develops a venous channel for the forebrain and the midbrain, as will be seen 

 in plate 2, figure 2, from a chick of 16 somites, which is the stage when the blood 

 begins to circulate. The superficial plexus opposite the forebrain and the mid- 

 brain arises, for the most part, from the deep plexus (text-fig. 1), but I have 

 also injected a few tiny connections between the superficial plexus and the aorta 

 itself in early stages. These, however, disappear and the superficial plexus drains 

 only the deep plexus. 



The vein which develops within the superficial plexus is characteristically 

 placed, and is very adequately shown by Evans for the stages of 17 to 25 somites 

 (Anatomical Record, 1909, III, figs. 3 to 6). At the stage of 29 somites this 

 primitive cerebral vein is clearly shown in plate 6. Owing to the flexure of the 

 midbrain, the primitive cerebral vein (v. cap. p. 1) runs directly across the 

 thalamus; and it receives a very interesting series of branches. It is obvious 

 that a very large number of the primitive veins opposite the cerebrum drain the 

 eye. Beginning with the position of the Gasserian ganglion, as seen in plate 6, 

 there is a plexus of veins which I have called the primitive maxillary veins (v. m. p.), 

 which drain the inferior part of the eye and the most anterior border of the cere- 

 brum. These veins have usually been called the primitive inferior ophthalmic 

 veins, and, according to the function which they actually perform at the stage of 

 plate 6, this would be perhaps a more logical name. However, the stage when 

 this plexus drains mainly the eye is very transitory. Soon the capillaries of the 

 maxillary arch develop and the plexus of veins which, at the stage of plate 6, 

 clearly lies in the maxillary arch, drains all of the structures of that arch, the roof 

 of the mouth, and the nose. The position of the maxillary vein and its corre- 

 sponding artery in the maxilla is shown for the pig in plate 7. In the chick of 

 the fourth and fifth days of incubation this group of veins clearly drains the entire 

 maxilla and receives branches from the most anterior part of the cerebrum and a 

 group of inferior ophthalmic veins, of which one of the most important runs in the 

 optic stalk. Therefore I have preferred to limit the name primitive inferior 

 ophthalmic veins to the branches of the primitive maxillary vein instead of calling 



