12 THE DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



structures. The intermediate portions of the mesh maintain the communications 

 between the deep capillary sheet and the superficial main channels. The few 

 communications of the aortic system with the intermediate and deeper jxart of 

 the plexus are maintained and unite with selected loops of the plexus to form slen- 

 der continuous channels which are eventually lost in the plexus. In this way the 

 irregular plexus of proUferating endothelial tubes is resolved into a definite circula- 

 tory system, consisting of a capillary mesh that is fed by deep branches from the 

 aortic system and drained by superficial tributaries into a main channel that 

 extends backward to empty into the common vitello-umbilical vein. By the time 

 that this is accomplished the contained blood is already slowly circulating through 



this primitive system. 



The main drainge-channel which thus becomes established on each side of the 

 head was originally called the anterior cardinal vein until it was recognized that 

 only the caudal portion of it properly belonged to the cardinal system. Evans 

 (1912, p. 676) suggested, as more appropriate for it, the name "primitive head- 

 vein,'' and the same term was utilized by Sabin (1917a). This primary head- vein 

 develops essentially in the same manner both in the chick and the pig— that is, 

 through the elaboration of a simpler and more direct channel through the superficial 

 part of the proliferating plexus. 



The difference, however, in the anatomy of chick and pig embryos is associated 

 with some difference in the details of the development of this primary head-vein. 

 The pre-trigeminal portion of it is identical in both forms and consists of a super- 

 ficial main channel, receiving tributaries from the deeper part of the plexus. It 

 forms in the interval between the thalamus and the optic bulb and leads backward, 

 median and ventral, to the trigeminal ganglion, where it temporarily connects with 

 the primordial system. The middle portion of the i)rimary head-vein, the portion 

 in the interval between the trigeminal and vagus nerves, is formed later than 

 the pre-trigeminal portion, and is formed relatively later in the chick than in the 

 pig. In the chick, it has been shown by Sabin (1917o) that the primordial channel 

 running along the ventro-lateral margin of the hindbrain persists, in embryos of 

 29 somites, as a single large channel communicating in front with the elaborate 

 plexus of the midbrain region and the already formed pre-trigeminal portion of 

 the main drainage channel. Caudally it communicates through the transverse 

 vein of the first intersixice with the anterior cardinal vein, the latter forming the 

 caudal portion of the primary head-vein. Thus, in the chick the hindbrain portion 

 of the primordial blood-channel (the vasa primitiva rhombencephah of Sabin) 

 serves temporarily as a part of the circulatory apparatus before its proliferative 

 function has been completed, but in the stage to which we are referring proliferating 

 loops have hegun to spread from the main channel over the wall of the hindbram 

 and its attached ganglia. Derived parti}- from these and partly from the plexuses 

 of the gill-arches, there is formed a series of superficial loops which link them- 

 selves together into a slender longitudinal channel communicating in front with 

 the pre-trigeminal portion of the main drainage-channel and extending backward 

 lateral to the otic vesicle, to empty into the anterior cardinal vehi. This consti- 



