14 THE DEVELOPMENTAL ALTERATIONS IN THE VASCULAR SYSTEM 



anastomosing loops into the more superficially placed primary head-vein, and thereby 

 are connected with the duct of Cuvier and the venous end of the heart. It is this 

 primary arrangement that exists in human embryos 4 mm. long, and this is the 

 earliest stage that was examined in connection with the present study. In figures 

 22 and 23, plate 2, are shown the left and right profiles of a model made by the Born 

 construction method of such an embryo (Carnegie Collection, No. 588, 4 mm. 

 long). This is shghtly younger than the stage shown by Mall (1905) in his figure 3, 

 although the conditions in the two are very similar. It is distinctly younger 

 than both the Ingalls (1907) and the Elze (1907) specimens, although they also 

 show the same primary type of circulation. In figures 22 and 23 the neural tube, 

 eye-stalk, trigeminal ganglion, and ear-vesicle are shown in gray and the recon- 

 struction was planned so as to show the relation to these structures of the main 

 arteries and veins, the latter being colored red and blue respectively. On ex- 

 amining these figures it is to be remembered that only the definite supply and 

 drainage channels are shown. In order to complete the system one must imagine 

 the neural tube as almost completely ensheathed by a capillary sheet into which 

 the arterial feeders open and from which the small venous tributaries arise. In 

 figure 23 fragments of this capillary sheet showing this relation to the drainage- 

 vessels are indicated in the hindbrain region. 



The morphological details of the models will perhaps be more readily under- 

 stood by the study of these two figures, in which the attempt has been made to 

 indicate the form and relations as clearly as possible, than by the following of a 

 descriptive text. Attention, however, may be directed to a few of the general 

 features. In the first place, it seems to be the rule that, throughout the whole 

 body of the embryo, the source of blood-supply has a central position and its flow 

 to the tissue is in a peripheral direction, whereas the return drainage system lies 

 more superficially and the flow is consequently in a central direction. As for the 

 drainage of the head, there is provided a simple and adequate channel, the pri- 

 mary head-vein. Its only deflections from a perfectly direct course are those 

 rendered necessary by the structure of the parts. Its position is affected by the 

 trigeminal, facial, glossopharyngeal, and vagus nerves, due to their respective 

 placodal relations to the ectoderm. It could not pass lateral to the trigeminal 

 mass, and so is deflected inward. The facial and glossopharyngeal gangha Ue in 

 positions sufficiently ventral so as not to interfere with its superficial position; the 

 former, however, deflects it upward slightly. The ganglion nodosum of the vagus 

 lies dhectly in its course and, as in the case of the trigeminal, the channel is thereby 

 forced to take a median course, since the preferred superficial course is ruled out 

 by the placodal attachment. We thus meet with a median deflection at that 

 point, so that the primary head-vein curves caudally around the vagus trunk. 



Just as the main trunk of the primary head-vein follows the most simple and 

 direct course possible, so its plexiform tributaries are favorably situated for draining 

 the various areas which are present at that time. Those from the capillary sheet 

 of the brain-tube join it for the most part on its dorsal and medial border. In 

 accordance with the topography of the region, these tributaries are arranged in 



