84 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



to the Gasserian ganglion, but lies against the hindbrain; the primary head-vein 

 is also mesial to the ganglion, but lies ventral to the hindbrain. 



On the other hand, the new capillaries pass dorsal to the placode of the 

 acoustic complex, and the slight dorsal curve of the primary head-vein (which is 

 very evident in plate 6) indicates this adjustment of the vein. The placode of 

 the acoustic complex is indicated in plate 6 by a film over the primitive vessel of 

 the hindbrain opposite the root of the eighth nerves. The vena capitis prinia 

 passes ventro-lateral to the otic vesicle and again curves slightly dorsalward 

 opposite the ganglion of the glosso-pharyngeus. 



In another injected specimen of this stage the superficial vein is a slender 

 capillary plexus spanning the gap between the second aortic arch and the anterior 

 cardinal vein, and not yet connecting with the veins of the maxillary arch. Thus 

 this middle segment of the vena capitis prima (the so-called vena capitis lateralis) 

 begins as an irregular capillary plexus between the aortic arches and the anterior 

 cardinal vein. It becomes a true head-vein, in the sense that it drains the entire 

 head, whereas the primitive vascular channel of the hindbrain (vena capitis 

 medialis) is a true neural vessel draining the brain only and not the entire head. 



Up to the stage when the capillaries of the visceral arches develop, the primi- 

 tive channel of the hindbrain serves as the only drainage channel in the head, 

 and this means practically for the brain alone; but as more structures in the head 

 differentiate, a new vascular channel develops to drain these structures. This 

 new chain of capillaries which receives the blood of the primitive cerebral vein 

 by means of the relations of the maxillary veins is so direct and so favorable a 

 channel for the blood of the primitive cerebral vein that the vena capitis prima 

 develops very rapidly at the expense of the primitive vessel along the hindbrain. 



By far the most interesting way to follow this transformation is by watching 

 the living chick. As is very clearly shown in plate 6, there is a stage when there 

 are two venous channels for the head of the embryo a large, deep channel along 

 the hindbrain and a superficial tiny capillary chain farther ventral and farther 

 lateral. While this more lateral channel is very tiny, it is hard to see it in the 

 living chick, because there are few if any blood-corpuscles in it, and it is by the 

 injection of blood, as it were, that one sees the vessels. In one chick opened 

 toward the close of the third day and kept in a warm box, the two veins were of 

 equal size when first observed, but in the course of about 2 hours the ventral 

 channel had become by far the larger. This important change can be followed 

 in the living chick either by opening a number of eggs at the close of the third 

 day of incubation and observing the veins by the blood within them or by keeping 

 a single chick of the right stage under observation for 4 or 5 hours. 



In such living specimens it can be seen that the deep vessel of the hindbrain, 

 which remains as a single vessel for 2 days, becomes a capillary plexus as soon as 

 the mass of venous blood from the forebrain and midbrain becomes shunted 

 through the superficial vein. In an injection many interesting details of this 

 process can be made out which are not so clearly seen in the living chick. In a 

 stage still earlier than that shown in plate 6, the deep vessel of the hindbrain 



