DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 107 



plate 1, figure 1. It shows the relative size of the optic vesicle and the forebrain 

 at this stage. The side of the thalamus and the midbrain is nearly covered by 

 a plexus extending toward the dorsal wall of the neural tube. Along the cephalic 

 part of the hindbrain is a wide vessel connected with the aorta by two arteries. 

 It already shows sprouts along its dorsal border, two of which bound the otic 

 vesicle. This deep single channel becomes a plexus along the side of the neural 

 tube at a point just in front of the first myotome. This is the point where the 

 cephalic end of the anterior cardinal vein joins the neural vessels, and, in terms 

 of the neural tube, it is at the cephalic end of the origin of the roots of the vagus 

 nerve. The transverse vessel of the first interspace which is so prominent in the 

 chick is but a small vein in the pig like the other intersegmental veins, and does 

 not become an important vessel, as in the chick. As is well known, the upper 

 myotomes are occipital myotomes, so that it is clear that the point of transition 

 between the deep vessel of the hindbrain and the primitive plexus, as shown in 

 plate 1, figure 1, is not between the hindbrain and cord, but is near the upper 

 part of the medulla. The lateral plexus along the cord is injected in the specimen 

 of plate 1, figure 1, down to the fourteenth somite, which is opposite the lowest 

 transverse artery injected, and the spinal arteries are injected down to the 

 twentieth interspace. These lower vessels are omitted in the drawing. 



In plate 1, figure 1, can be traced very clearly the origin of the cephalic part 

 of the primary head-vein; that is, the primitive cerebral vein. Extending from 

 the groove between the telencephalon and the diencephalon (as Evans showed in 

 his figure 395 in 1912), is a superficial capillary plexus, indicated in blue, which 

 receives its blood from the deep plexus of the forebrain and midbrain and drains 

 into the deep vessel of the hindbrain. In this plexus will develop the primitive 

 cerebral vein; at this stage it is entirely a plexus without any definite longitudinal 

 channels. The specimen is just at the stage of the second vascular arch, which is 

 probably present and uninjected, as shown in Evans's figure 394 from an earlier 

 stage. Opposite the lower end of the primitive vessel of the hindbrain is a plexus 

 of exceedingly tiny vessels spanning the gap between the deep vessel of the hind- 

 brain and the anterior cardinal vein on the one hand, and reaching toward the 

 second aortic arch on the other. These tiny capillaries form the origin of the 

 lateral vein of the region, that is, the middle segment of the primary head-vein, 

 just as has been shown for the chick. This plexus will span the gap between the 

 second and third aortic arches as they form, and the cephalic end of the primary 

 head-vein, until there is a double vascular channel from the head, as shown on 

 plate 4, figure 1. 



Figure 1 of plate 4 is from a specimen of the same litter as that of figure 2 of 

 plate 5, and is given because of the extravasation in the head region in the latter 

 figure. At the stage of three aortic arches the primary head-vein is complete. 

 The primitive veins which pass ventral to the eye are not injected in the specimen 

 of plate 4, figure 1, except just where they join the primary head-vein in front of 

 the ganglion of the trigeminus. The primary head-vein starts opposite the 

 thalamus and extends in a double curve down to the anterior cardinal vein. It 



