DIRECT GROWTH OF VEINS BY SPROUTING. 5 



mesenchyme. Buell was unable to find any clumps of angioblasts unconnected 

 with the main mass, so he had no evidence of a direct differentiation of these cells 

 from mesenchyme; rather, they seem to come directly from the wall of the 

 sinus venosus; but lie had abundant evidence that the period of origin of the 

 pulmonary vessels falls well within the angioblastic stage of the vascular system. 

 This mass of angioblasts forms at a stage when the lung-bud lies directly dorsal 

 to the sinus venosus. The cells spread over the surface of the gut, making a 

 plexus which connects with the dorsal aorta, the ventral aorta, and both cardinal 

 veins. By the liquefaction of their cj'toplasm the plexus of angioblasts becomes 

 a plexus of vessels. The pulmonary veins form in the angioblasts that are directly 

 connected with the sinus venosus, while the arteries form in the more dorsal loops 

 of the post-branchial plexus, the formation of the pulmonary artery slightly preced- 

 ing the completion of the pulmonary arch. Thus the fundamental morphology of 

 the vascular system of the lung in the chick is established. 



This volume also contains a study by Miss Finley of another phase of this prob- 

 lem. She has studied the invasion of the subcutaneous tissue of the head of the 

 human embryo by the vascular system. In the head there are two primary vas- 

 cular plexuses: One in the meninges, the forerunner of the vessels of the central 

 nervous system, the meninges and the skull, which begins very early; the other 

 the subcutaneous plexus, which develops late. Its late appearance makes this 

 subcutaneous plexus a favorable place to study the problem of the differentiation 

 of angioblasts in a late embryonic or early fetal stage. Miss Finley has found 

 evidence of a progressive differentiation of angioblasts in front of an invading 

 zone of vessels. There are four zones, beginning at the periphery: (1) An avas- 

 cular area, with undifferentiated mesenchyme. (2) A zone in which the vascular 

 system consists of a massive plexus of cells. This vascular plexus, interestingly 

 enough, consists very largely of masses of red cells, with a somewhat incomplete 

 endothelial border, so that the observations have a very important bearing on the 

 method of origin of the red blood-cells in the mammal. The process is clearly an 

 intermediate one between the condition found in the chick, where the red cells arise 

 within vessels, and a process of a diffuse origin of red cells which would subse- 

 quently have to migrate into vessels. These observations will be of especial 

 value in the restudy of mammalian bone-marrow, where the question of the 

 relation of the origin of red cells to endothelium has not been satisfactorily cleared 

 up. Along the edge of this angioblastic zone are a very few isolated masses or 

 chains of angioblasts. Miss Finley has studied the tissue, first in place and then in 

 total preparations, stripped from the head of the embryo, so that she is sure of the 

 very small number of such isolated clumps. (3) The third zone, which is formed 

 from the second, consists of capillaries, some of which are empty, while some contain 

 red cells. This zone probably does not have any circulation. (4) The fourth zone, 

 leading to the neck, has definite vessels in which one can make out a pattern that 

 may persist. Thus she has demonstrated an advancing zone in the angioblastic 

 phase, definitely related to the formation of red cells, in human embryos about 

 30 mm. long, corresponding to the end of the second month of pregnancy. 



