DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



such as are shown in plate 3, figure 5. It develops early and is very straight. In 

 the chick it is not straight and therefore is much less striking in total prepara- 

 tions. Its primary connections with the posterior cardinal vein are, lateral to 

 the Wolffian duct, as seen in text-figure 5 for the pig. That this is also true for 

 the chick is shown by Graefe's figure 6 (1906), which shows the pronephros of the 

 chick at the stage of 2 days and 15 hours. Later, in both the pig and chick, these 

 two veins are connected by branches which are mesial to the duct, as shown in 

 text-figures 7 and 8. 



The failure to take into account the lateral cardinal veins has led to some 

 confusion in the literature; for example, in the study of the pronephros, Graefe 

 (in his figure 11) has labeled the lateral vein close to the Wolffian duct the sub- 

 cardinal, while in figure 13 he has labeled the true subcardinal vein ventral to the 

 nephritic artery the subcardinal, but has not labeled the lateral vein at all, though 

 it is shown in the section. 



In Keibel and Mall's Embryology, Felix gives some extremely interesting 

 sections from the R. Meyer human embryo No. 300. This embryo had 23 somites 

 and was 2.5 mm. long. It is to be compared with my plate 1, figure 1. In figure 

 532 a Felix shows solid angioblasts, both dorsal and ventral to the Wolffian duct; he 

 does not label the dorsal angioblasts which represent the posterior cardinal vein, 

 but on the other hand calls the ventral angioblasts the posterior cardinal vein. 

 Again, in figure 559 he calls angioblasts which are ventral to the duct the posterior 

 cardinal vein. These sections show that in the human embryo there are angio- 

 blasts both dorsal and ventral to the duct and bring out the value of the two 

 names for the veins, the posterior and lateral cardinal veins. They also show 

 that the posterior and lateral cardinal veins extend as solid angioblasts and so 

 bring up the question as to whether these veins may not differentiate as chains of 

 angioblasts connected with the aorta by chains of angioblasts. 



CONCLUSION. 



In this study it seems clear to me that the chick affords very valuable material 

 for the study of the most fundamental point in connection with the vascular 

 system that is still at issue, namely, how long in the life of the embryo do new 

 angioblasts continue to differentiate from mesenchyme and join the blood-vessels? 

 The answer to this question involves more extensive observations on the living 

 blastoderm than I have yet made. It has been shown that blood-vessels first 

 arise not only in the membranes but also in the embryo by a differentiation of 

 cells into angioblasts, by the process which His had described, and not from a 

 dilatation of spaces in the mesenchyme and a flattening-out of cells to form their 

 border. 



It has been proved that the aorta at least in part differentiates in situ. Evi- 

 dence has been given that a part at least of the neural vessels and their connec- 

 tions with the aorta differentiate in situ. On the other hand, the cardinal veins 

 begin as a growth from the wall of the aorta. They are a longitudinal anastomosis 

 between direct branches of the aorta. A more detailed study of the later stages of 

 the cardinal veins is necessary to determine if any part of them differentiates in situ. 



