88 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



In the transition from the stage in which the primitive channel of the hind- 

 brain serves as the vein of the brain to the stage when the new lateral superficial 

 vessel the true primary head-vein is complete, it is clear that the primitive 

 transverse vein of the first interspace is cut out of the main line of drainage for 

 the head. It does not form a part of the primary head and neck vein of the 

 embryo. Thus the primary head-vein, from the standpoint of development, 

 consists of three parts: an anterior division, which is the primary cerebral vein; a 

 second portion, which is a true head-vein draining the entire brain, forebrain, 

 midbrain, and hindbrain, as well as the visceral arches; and thirdly, the anterior 

 cardinal vein. The transverse vein of the chick persists as a root of a character- 

 istic vein of the hindbrain namely, a vein which arches caudalward along the 

 lateral surface of the medulla. This vein of the medulla will be followed farther 

 in the pig. It was called the posterior cerebral vein by Mall. The position of 

 the transverse vein of the chick embryo in the first interspace is also just opposite 

 the cephalic end of the ganglia of the vagus nerve. As soon as the superficial 

 vein the primary head-vein is formed, the vascular channel of the neck 

 straightens out, and there is then no longer any way of distinguishing the exact 

 place where the second segment of the primary head- vein joins the third segment 

 or the anterior cardinal vein, for the two become a single, continuous channel. 

 From now on, the place of transition can be indicated only in a general way by 

 the root of that vein of the medulla which follows the roots of the vagus nerve 

 along the medulla; and it is well known that veins are shifting landmarks. Stated 

 in other words, the anterior cardinal vein extends along the entire zone of the 

 occipital myotomes, and as the occipital muscles develop these myotomes become 

 indistinct landmarks. 



The first interspace is thus a transitory landmark, and in later stages and as 

 soon as the superficial head-vein connects directly with the anterior cardinal vein 

 and eliminates the transverse vein of the first interspace from the direct line of 

 drainage for the blood of the brain, the distinction between the head-vein and 

 the anterior cardinal vein becomes less obvious. The cephalic portion of the 

 head-vein develops to drain the forebrain and midbrain; the middle portion 

 develops to drain the brain and the gill-arches. The vein of the anterior part of 

 a chick of the fourth day of incubation is therefore a composite structure, so far 

 as development is concerned. However, at the fourth and fifth day of incuba- 

 tion there is a single long vein extending from the groove between the cerebrum 

 and the thalamus down to the duct of Cuvier. This vein receives branches from 

 all the various structures of the head. The neural branches come from the cere- 

 brum and the eye from the thalamus, the midbrain, and the hindbrain. The 

 branches from the hindbrain are especially modified by the ganglia of the hind- 

 brain and the otic vesicle. On the ventral aspect this vein receives branches 

 from the developing visceral arches and from the somatopleure opposite the heart. 

 The entire vein may thus be called the embryonic head-vein, or the vena capitis 

 prima. 



