86 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



is one on each side. In plate 6 this vessel is labeled the basilar artery (a.b.), 

 which is an illustration of the fact that the relations of the arteries of the adult 

 may have too great an influence on the naming of the embryonic vessels. This 

 vessel is not even a capillary which will become the basilar artery, because it is 

 not in the mid- ventral line; it is rather a vessel which will become a part of a 

 capillary plexus that will gradually reach the mid-ventral line, where the basilar 

 artery will form. At the stage of plate 6 there are bilateral longitudinal arteries 

 along the thalamus, the mid-brain, and the hindbrain, as can be proved by a 

 direct ventral view of the specimen. The relations and the importance of this 

 vessel would be emphasized by calling it a part of the primary longitudinal neural 

 artery. On the other hand, the vessel shown in plate 7 from a pig embryo of an 

 older stage is in the mid-ventral line and is thus the true basilar artery. 



During the fourth day of incubation the longitudinal artery seen opposite 

 the first, second, and third somites in plate 6 grows caudalward along the ventro- 

 lateral surface of the spinal cord on either side, to the caudal end of the neural 

 tube. These ventro-lateral arteries develop as a longitudinal anastomosis between 

 all the segmental arteries of the spinal cord. At the stage of the fourth day 

 of incubation it is clear that the vascular plexus along the entire surface of 

 the neural tube is supplied with blood by bilateral ventro-lateral arteries which 

 extend from the groove between the cerebrum and the thalamus to the caudal 

 tip of the tube. These two longitudinal arteries are originally in the form of a 

 plexus on either side of the subthalamus, as is still better shown in plate 7 for 

 the pig, and are more definitely a single channel along the rest of the course. 



This longitudinal neural artery receives its blood from the forerunner of the 

 carotid arteries on either side and from the segmental arteries. It is easy to see 

 that it is these important longitudinal arteries which will ultimately give rise to 

 the circle of Willis, the basilar artery, and the anterior spinal artery. 



The development of the anterior spinal artery has been worked out in the 

 pig by Evans (1909 and 1912). In the chick the anterior spinal artery does not 

 form until the fifth day of incubation. During the fourth day there are two 

 ventro-lateral arteries along the spinal cord which are placed on either side of the 

 notochord and are not connected except by an occasional capillary across the 

 mid-ventral line; they make a sharp ventral boundary for the lateral plexus on the 

 spinal cord. These two longitudinal arteries are just mesial to the point where 

 the spinal arteries meet the spinal cord, as can be seen in Evans's figure 437 c in 

 the "Manual of Human Embryology" (Keibel and Mall). They give rise to the 

 characteristic anterior arteries which penetrate the spinal cord. During the fifth 

 day of incubation these two longitudinal arteries become connected with each 

 other across the mid-ventral line, which is the beginning of the formation of the 

 anterior spinal artery. The stage of the fourth day of incubation for the chick 

 in which there are bilateral longitudinal arteries along the ventro-lateral border 

 of the entire neural tube from the point of origin of the carotid artery to the tip 

 of the spinal cord is an important stage for understanding the blood-supply of 

 the nervous system. 



