OF THE BRAIN OF THE HUMAN EMBRYO. 9 



introduced that results in the obUteration of a considerable part of the largest vein 

 of the head. This is compensated for by a new channel which takes a more dorsal 

 course, and which eventually forms the sigmoid portion of the lateral sinus. 



Finally, under the fifth period we would include the late histological changes in 

 the walls of the vessels that convert them into the adult arteries, veins, and the 

 various types of sinuses. These histological factors, however, are not considered 

 in the present paper and are merely mentioned to complete the sequence, as they 

 have no determining influence on the phenomena of the preceding four periods. 



The observations that are reported are chiefly concerned with the third and 

 fourth developmental jjeriods; that is, after the primary circulation of the head is 

 established and during the cleavage and adjustmental stages. A review, however, 

 will be made of the history of the vascular system of the head previous to that 

 time, which will be based in large part on the important observations of Evans 

 (1909, 1912) and Sabin (1915, 1917f/, 19176). The study of the development of 

 the vascular system of the head of the human embryo was initiated in this laboratory 

 by Professor Mall (1905). Later I continued the same investigation and reported 

 (Streeter, 1915) some of the features of the adaptive metamorphosis of the dural 

 veins. In the present paper I shall include much of the same subject-matter and 

 incorporate with it further observations made on additional material, thus making 

 it possible to treat the general subject more completely and to demonstrate the 

 relations of the dural veins to the principal arteries of the head. 



ESTABLISHMENT OF PRIMORDIAL VASCULAR SYSTEM OF HEAD. 



A distinct advance in our knowledge of the origin of the vascular system has 

 recently been made by Professor t^abin through the study of living preparations 

 of the growing chick. By that means it was possible to observe directly the prin- 

 cipal steps in blood-vessel formation. Certain steps in the process that were still 

 under dispute have thus become established, as well as others which are new 

 observations. According to her description (Sabin, 19176), the vaso-formative cells 

 or angioblasts are differentiated from the mesoderm, and on proUferation they form 

 small, dense, syncytial masses which join one another by means of tiny processes 

 of cytoplasm. In this way there are formed plexuses of solid 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 a liciuefaction of their 

 cytoplasm occurs in such a manner as to convert the sohd angioblastic cords into 

 vessels filled with clear fluid. This is brought about by a process of vacuolization 

 and the formation of an "intracellular" lumen. Vacuoles appear near the nuclei 

 and rapidly enlarge, so that in from one to two hours there occurs a complete de- 

 struction of the cytoplasm and nuclei of the central i^art of the angioblastic cords, 

 resulting in the formation of a clear plasma, the periphery of the angioblastic mass 

 being jireserved as an endothelial boundary. The liquefaction of cytoplasm occurs 

 both in the loops of the angioblastic plexuses and in masses of angioblasts 



