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JOHN LEWIS BREMER 



regularly, and the meshes are slightly smaller, especially toward 

 the head. Closer examination will show that each cord extends 

 from the lateral heart almost directly toward the median line, 

 then suddenly spreads longitudinally, as though its further direct 

 course were blocked by some obstruction. In all but two places, 

 which are marked by the arrows, the longitudinal strands of this 

 mesial net have anastomosed with others; at these two places 

 the longitudinal network is interrupted, and we may see clearly 

 that this part of the net, the future dorsal aorta, is an ingrowth 

 from the lateral vessels. 



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Fig. 2 Rabbit, 85 days, H.E.C. no. 650, section 317. Section through the plane 

 x - y in fig. 1. Mil. dr. = medullary groove ; Som. = 5th somite; Ent. = entoderm. 

 11, b. c, etc. = sections of the angioblast network. X 175. 



If we examine fig. 2, a section of the same embryo, we can see 

 these cords in their relation to the germ layers. The embryo has 

 become shrunken in preservation, so that the layers are separated 

 slightly from one another. The angioblast cords, indicated by 

 the small letters, are seen to adhere now to the entoderm, now to 

 the mesoderm, a fact which made them hard to follow, but which 

 is obviously the result of this shrinkage. Let us imagine that 

 the layers are close together. It will be seen that the somite will 

 then touch both the ectoderm and the entoderm; it is this that 

 forms the obstruction to further growth of the angioblast cords 

 toward the median line. Where the somite does not exist, as in 

 the head region, the cords extend further, and are then halted 

 by the closely approximated medullary groove and entoderm; 



