WITH SPECIAL REFERENCE TO THE VASCULAR SYSTEM. 87 



In the body-stalk and chorion the process of vessel formation, by accretion and 

 accessions from without, may already have been superseded by the direct growth 

 and extension of preexisting vessels. In the yolk-sac and embryo both processes 

 are still going on hand in hand. There remains, however, much to be done in the 

 way of linking up, not only the intraembryonic vessels with each other and with the 

 heart, but also in establishing relations between the intrasomatic and extrasomatic 

 channels by way of the heart, aortae, and umbilical veins. There is an unmistakable 

 precocity in the vascular development in man, a certain element of haste almost, 

 so that after a given stage has been reached the ground plan of the embryonic cir- 

 culatory system is sketched out with all possible expedition. Obviously, the most 

 promising method of procedure would be to begin construction at every suitable 

 point. The paramount importance of an adequate circulatory mechanism, as well 

 as its extreme susceptibility, is too well known to call for comment. It is likewise 

 unnecessary to rehearse the evidence that the vessels of the embryonic body are 

 largely formed in loco instead of by any invasion from without (Halm, 1909; Miller 

 and McWhorter, 1914; Schulte, 1914; Reagan, 1915; Sabin, 1917). 



In the particular case in question three main groups of embryonic vessels may 

 be recognized — the heart, aortae, and umbilical veins, each group completely isolated, 

 only the aorta having a connection with the extraembryonic vessels and this with 

 certainty only on one side. The aorta and umbilical vein are both still in process of 

 formation at a number of separate points. To what extent this might also have 

 been true for the heart and first arches we can not say, but doubtless the principles 

 involved were essentially the same. Conditions here are such that the many con- 

 nections still required to complete the vascular circuit may be made within a very 

 short period of time. It would be useless to speculate concerning priority or 

 sequence, since there must be considerable variation; but in any case the union of 

 the intraembryonic and extraembryonic portions of the umbilical vein would appear 

 to be a relatively late event. 



The vascular system, as we find it in this embryo, is really of little more impor- 

 tance to its possessor at this particular moment than are the future respiratory and 

 excretory systems, of which there are as yet not even the faintest traces. The 

 special organs for these last-named functions will appear in their own good time. 

 In the meantime, however, these and other functions will be discharged in other 

 ways, but even now, as later, only upon the basis of an adequate circulatory mechan- 

 ism. At first the modest needs of the embryo, as regards a circulating or diffusible 

 medium, will be met by the relatively great surface which its component parts 

 present to the surrounding fluid, as in the yolk-sac and ccelom. There can be 

 no doubt that important nutritive material finds its way to the young embryo 

 through the ccelom, or secondarily through the yolk-sac. Without saying that the 

 fluid within the umbilical vesicle differs materially from that outside, one may still 

 conceive that its lining entoderm may play some small, though evanescent role, 

 and that its mesoderm and blood-channels have something more to do than the 

 formation of early blood-cells and vitelline vessels. 



In addition to the extensive absorptive or excretive surfaces offered by the 

 early embryo, we have a second intermediary mechanism for adequate diffusion in 



