ORIGIN OF THE POSTERIOR LYMPH HEART 421 



of the mesenchymal cells near by, in the direction in which this 

 'sprout' would extend, show a tendency to become elongated {19), 

 but they are separated from the endothelial cell by indifferent 

 mesenchymal cells, and their nuclei are quite chromatic. They 

 probably represent cells which are about to flatten and to limit 

 a plasmatocyst. 



Since disconnected plasmatocysts have been found; since 

 all gradations between an indifferent mesenchymal cell and a 

 typical endothelial cell have been observed; and since, in the 

 section just described, we find most distally an uninjected plas- 

 matocyst, containing a differentiating blood cell, then a single 

 endothelial cell enclosing no lumen, and finall}^ a lumen con- 

 nected with the veins into which the injection mass has entered, 

 it does not seem justifiable to call this structure a venous sprout. 

 It should rather be considered as a plasmatocyst which has 

 differentiated in situ, and connected secondarily with the vein. 

 Whether the endothelial cells between the plasmatocyst and 

 the vein arise by an in situ differentiation, or by a mutual growth 

 of the plasmatocyst and the vein toward each other, it is im- 

 possible in this particular case to determine definitely by the 

 study of sections or injections. The latter interpretation would 

 in no way invalidate the fundamental conception that endothelium 

 arises in situ from mesenchyme. It merely implies that en- 

 dothelial cells once formed are capable of proliferation, as cells 

 in general are. It should be noted that discontinuity of the 

 lumen of the 'sprout' present in figure 7 shows clearly the utter 

 inadequacy of the injection method for demonstrating all of 

 the endothelium in the embryo. 



As regards the further development of the blind spaces in 

 the mesenchyme, we have seen that in the 10.5 and 11 mm. 

 embryos there exist a number of spaces in the mesenchyme just 

 lateral to the caudal muscle plate, and that these spaces are 

 bounded by mesenchymal cells. Some of these spaces are con- 

 nected at this stage with the lateral branches of the coccygeal 

 veins, and certain blood cells, differentiating from the mesen- 

 chyme, have become included in some of the disconnected 

 spaces. In the 12.5 mm. and 13.5 mm. embryos more and 



