146 GEORGE L. STREETER 



Acknowledgment should furthermore be made to Professor Sabin 

 and Professor Evans whose valuable experience in the study of 

 the vascular system and whose unique preparations were gener- 

 ously placed at the writer's disposal. Under these conditions the 

 opportunity has been especially favorable for clearing up some 

 of the obscure factors in this interesting field. It is also be- 

 lieved that, in addition to any interest the reader may have in 

 the morphological details of the veins themselves, his attention 

 will be attracted by the bearing they have on the broader prin- 

 ciples involved in the establishment of vascular drainage. On 

 following through the successive stages, which are to be outlined 

 in the following paper, it will be noted that they afford striking 

 examples of embryonic change and repeated adjustment of the 

 drainage channels consequent upon the alterations in the form 

 and condition of the particular area drained. Nowhere do we 

 find a clearer picture of these adjustment phenomena than in the 

 region of the brain. Its marked change in form and especially 

 the prolonged relative growth of the cerebral hemisphei'es make a 

 continuous series of alterations of the veins necessary that extend 

 far into the late embryonic stages. 



MATERIAL AND METHODS 



The material on which this study is based consists chiefly of 

 serial sections of human embryos taken from the Mall Collection, 

 which now forms the nucleus of the Department of Embryology 

 of the Carnegie Institution. In many of the specimens the blood 

 vessels had been previously injected with India ink or Berlin 

 blue or both. In one instance, No. 458, 54 mm. long, a total 

 preparation of an injected specimen was made and cleared in oil 

 and drawings were made directly from the specimen with the 

 camera lucida. The principal stages, however, were usually 

 based on profile reconstructions prepared from serial sections. 

 These will be specified under their individual descriptions. In 

 some cases portions of the veins were modeled after the Born re- 

 construction method. The oldest stage examined was in an 

 embryo (No. 234 a) 80 mm. crown-rump length. The youngest 

 stage was an embryo (No. 588) 4 mm. greatest length, at which 



