DEVELOPMENT OF PRIMITIVE BLOOD-\ KSSI1LS. S.) 



begins to show very characteristic dorsal branches which conform to the surface 

 of the hindbrain and to the roots of its nerves. In fact, the first of these branches, 

 as can be seen in plate 6, tend to surround the root of the trigeminus, the root of 

 the eighth nerves, and the otic vesicle. While these branches of the primitive 

 vessel of the hindbrain are forming, the vessel itself also becomes a plexus. I 

 have injections which show how this takes place. At first the single channel 

 gives rise to a plexus of very large vessels which tend to run longitudinally, follow- 

 ing the pattern of the original vessel. Gradually the vessels of this plexus become 

 smaller and the longitudinal pattern is lost. I have not illustrated the develop- 

 ment of the plexus on the hindbrain for the chick, but this point is well shown 

 for the pig in plate 7, and the principles are the same in both forms. The plexus 

 on the hindbrain ultimately covers the hindbrain as completely as the plexus 

 on the midbrain shown in plate 6, but the pattern of the plexus is modified 

 by the structures of the hindbrain: (1) by the roots of the nerves and their sensory 

 ganglia; (2) by the otic vesicle, which for a time lies close to the hindbrain; (3) by 

 the special vascular structure of the roof of the fourth ventricle. As has been 

 said, the plexus into which the primary vessel of the hindbrain first breaks up 

 tends to have a longitudinal pattern; the ultimate plexus over the hindbrain, on 

 the other hand, tends, like the rest of the neural plexus, to show indistinct trans- 

 verse lines. This is, I think, plain in plate 7, and it leads to the subject of the 

 new arterial supply for the vessels of the hindbrain. 



A most important point in the history of the transformation of the primitive 

 vessel of the hindbrain concerns its relation to the neural arteries, and this point 

 is well brought out in plate 6. Taking into consideration the entire neural 

 tube, it is originally supplied by a series of arteries from the aorta: (1) a group 

 of vessels to the forebrain, that is, to the cerebrum and the thalamus, at the base 

 of the optic vesicle from the primitive arch of the aorta; (2) a few small arteries 

 opposite the midbrain; (3) a series of small arteries to the primitive vessel of the 

 hindbrain; (4) a series of intersegmental arteries, of which the most cephalic is 

 in the first interspace. In plate 6 an artery is shown on the right side from the 

 primary arch of the aorta, which is growing along the ventro-lateral surface of the 

 thalamus and the midbrain, and this artery is approaching a new artery, which is 

 at the same time growing forward along the hindbrain. This new artery is very 

 important; it starts as a longitudinal anastomosis along the neural tube between 

 the segmental arteries. In plate 6 it connects the first, second, and third segmental 

 arteries, which are occipital vessels, and is growing forward, making more and more 

 new connections with the deep vessel of the hindbrain. Plate 6 shows none of 

 the primitive arteries which connect the primitive vessel of the hindbrain directly 

 with the aorta; but in plate 7, from a pig of a still older stage, it is very interesting 

 to note that two of these original arteries still persist and take part in the forma- 

 tion of this new longitudinal artery. This longitudinal artery grows rapidly 

 forward until it joins the corresponding descending artery opposite the midbrain. 

 It is very clear in plate 6 that the longitudinal neural artery along the hindbrain 

 is originally along the ventro-lateral border of the hindbrain, and thus that there 



