106 CARNEGIE INSTITUTION OF WASHINGTON. 



ington, 1917. Dr. Sabin was able to observe in hanging-drop prepara- 

 tions of the Hving chick blastoderm the differentiation of angioblasts 

 from the mesoderm. She found that they form a plexus of solid bands 

 which could be distinguished from the surrounding mesoderm, owing 

 to the fact that they pass through rhythmic cycles of cell division. 

 During a part of this cycle they become very granular and opaque 

 and at that time stand out sharply from the intervening spaces occu- 

 pied by resting mesoderm. She was able, from such living prepara- 

 tions, to confirm by observations the experimental evidence of Hahn, 

 Miller and McWhorter, and Reagan, that angioblasts are differen- 

 tiated not only in the membranes, as originally described by His, but 

 also within the body of the embryo. The aorta is shown to arise, 

 in part at least, from angioblasts differentiating in situ from mesoderm, 

 but there is some evidence that this is also true of the primitive plexus 

 of vessels on the surface of the nervous system. 



Dr. Sabin's paper includes a detailed description of the primitive 

 blood-vessels of the chick and pig in which injections have been made at 

 earUer stages than have heretofore been published. The development 

 of the first true head-vein — the vena capitis prima — is described as 

 arising in three segments. The anterior segment, draining the fore- 

 brain and midbrain, originally empties into the primitive vessels or 

 capillary sheet of the hindbrain; the posterior segment, which even- 

 tually unites the other two, is derived from the capillary loops which 

 connect the visceral arches with the anterior cardinal vein. With the 

 establishment of this continuous lateral channel the primitive vessel 

 of the hindbrain is released from the function of draining the forebrain 

 and spreads as a pial plexus over the wall of the hindbrain, in which are 

 subsequently developed longitudinal arterial vessels. This establishes 

 the interesting fact that vessels which at one stage of their development 

 would be called venous, at a later stage may become arterial, and in 

 them the direction of the flow of blood is entirely changed. 



The subintestinal artery in the pig and chick is cited by Dr. Sabin 

 as another example of an entire change in function on the part of a 

 vascular plexus during the course of its development. The constant 

 alteration in the form and communications of the primitive vascular 

 system makes it necessary to redefine the term artery and vein, as 

 applied in the embryo. Instead of naming blood channels according 

 to the vessels for which thej form the primordia, Dr. Sabin recom- 

 mends naming them for the different stages, according to the functions 

 they perform at that particular time. 



A morphological study of the early development of the ureter and 

 the embrj'-ological aspects of the occurrence of double ureter has been 

 made by Dr. E. G. Davis. In collaboration with Dr. Hugh H. Young, 

 this work, together with a clinical study of the condition as occurring 

 in adults, has been published. 



