DEPARTMENT OF EMBRYOLOGY. 99 



The study of the development of the vascular system of the head 

 in the human embryo was initiated in this laboratory by Professor 

 Mall in 1905. Later I undertook the same investigation and reported 

 some of the features of the adaptive metamorphosis of the dural veins, 

 as referred to in our report for the year 1915. Since then it has been 

 carried farther and the results have been pubHshed in completed 

 form. In this paper it is pointed out that a primordial or precircula- 

 tory period exists, during which the vessels of the head are engaged 

 in differentiation and growth in the elaboration of a plexus. Their 

 form at this time is not influenced by conditions that would favor the 

 efficient circulation of the contained blood, and they are not to be 

 considered as arteries or veins, but rather as a germinal bed from 

 which arteries, veins, and capillaries are subsequently derived. Later, 

 as the circulatory flow of the blood becomes estabhshed, the blood- 

 channels respond by conforming to the hydrodynamic requirements 

 and the plexus is resolved into a capillary bed mth favorably situated 

 aortic feeders and simple and direct venous drainage channels. This 

 constitutes the primary type of the circulation in the head. The 

 paper then describes an interesting series of adjustments in the 

 arrangement of the blood-vessels in adaptation to the developmental 

 alterations in the form, size, and condition of the structures of the 

 head region, the process being followed by the attainment of the 

 adult arrangement. It is pointed out that the vascular apparatus is 

 continuously adequate and complete for the structures as they exist 

 at any particular stage; as the environmental structures progressively 

 change, the vascular system also changes and thereby is always adapted 

 to the newer conditions. 



A distinct advance in our knowledge of the differentiation of vascular 

 endothelium has recently been made by Professor F. R. Sabin through 

 the study of living preparations of the growing chick. By this means 

 it was possible to observe directly the principal steps in blood forma- 

 tion. Certain phases of the process that were still under dispute, as 

 well as others which were new observations, have thus become estab- 

 lished. According to Dr. Sabin's description, the vaso-formative cells 

 or angioblasts are differentiated from the mesoderm, and on prohfera- 

 tion they form smaU, dense, syncytial masses which join one another 

 by means of tiny processes of cytoplasm. In this way are formed 

 plexuses of sohd angioblastic cords, the growth of which is maintained 

 partly by proliferation of their constituent cells and partly by the 

 further addition of new angioblasts, which differentiate from the 

 adjacent mesoderm. During the formation of these angioblastic 

 plexuses there occurs a liquefaction of their cytoplasm in such a 

 manner as to convert the solid angioblastic cords into vessels fiUed 

 with a clear fluid. This is brought about by a process of vacuoHzation 

 and the formation of an intracellular lumen. Vacuoles appear near 



