The interest of the writer was first directed towards the study of the blood vessels of the head by comparing a dissection of the head arteries of the pig with one of the corresponding vessels in the pigeon. Both systems of vessels include a rete mirabile, but whereas that of the pig interrupts the internal carotid just before its entrance into the sella turcica, that of the pigeon corresponds to the temporal rete of the chick described in the present communication, and is found in the orbit in no immediate connection with the internal carotid. It was difficult to see how acommon functional explanation could be applied to both these retia. My attention was then drawn to the work of Shellshear, who from comparative studies of the distribution of the encephalic arteries in adult material had advanced certain hypotheses, summarized later in this communication, regarding the way in which blood vessels were supposed to develop. The most direct way of testing Shellsi-iear s hypotheses seemed to be that of studying intensively the development of the blood vessels in a single form. The examination of the literature of the development of the vascular system revealed the fact that hardly ever, if at all, had the development of arteries and veins been studied at the same time. Moreover, through the development of technique for the injection of embryonic material, it had been shown in many forms that a preliminary stage of capillary plexus is antecedent to definitive blood vessels. Yet although comparatively early embryos of birds and mammals had been studied by this means, no work had been done on any form in following the development of the blood vessels from the earliest stages to the adult condition. For the chick, Miss Sabin had given an account of the development of the blood vessels from the earliest stages up to the twenty-nine somite stage, Sabin (1917). It was decided to attempt to continue this account of the development of the blood vessels of the chick for the remainder of the embryonic period. I am grateful to Professor J. Stanley Gardiner, F. R. S. , and to Mr. C. Forster-Cooper for the privilege of working in the Zoological Department and the Museum of Zoology respectively of the University of Cambridge. I am further indebted to Mr. C. Forster-Cooper for much help and to Dr. S. M. Manton for her never failing advice and assistance in many matters of histological technique, and in the preparation of illustrations. Mr. G. R. de Beer was good enough to offer advice on certain matters, as was Mr. W. J. Heasman on the subject of the preparation of transparencies.