308 Papers from the Department of Marine Biology. 



endothelium appears, blood elements may originate. Again, if abnor- 

 mal conditions could explain endothelial proliferation and metamor- 

 phosis in embryos, then the experimental Fundulus embryos of 

 Stockard, in which cessation of heart-pulsation produced stasis in the 

 blood-vessels, and a consequent degeneration of some of the included 

 cells, should show endothelial proliferative activity; but according 

 to Stockard this endothelium is inactive. 



Emmel lays much stress upon the connection between certain ventral 

 aortic clusters and intra-arterial cell-masses. Vessels containing such 

 masses are interpreted as "degenerating"; but the evidence that such 

 are degenerating is unconvincing. In a 10-mm. pig embryo cut in 

 transverse section I found a large cluster attached to one side of the 

 mouth of the superior mesenteric artery. A similar condition prevails 

 in the 5-mm. mongoose embryo. These vessels are not degenerating; 

 if cut obliquely similar clusters might appear to be attached to an 

 intra-arterial cell-mass filling the vessel. Emmel's section No. 6 

 (p. 419) is apparently obliquely cut. The " intra-arterial cell-mass" 

 may be simply attached to one side of the mouth of the vessel. Such 

 conditions occur in both the mongoose and pig embryo, where no sign 

 of atrophy appears in the vessel itself. On the other hand, ventral 

 branches which later disappear can be seen free of cell-clusters. In 

 short, neither the degeneration nor the occlusion of all aortic branches 

 associated with clusters is definitely proved. 



Nevertheless, certain aortic branches undoubtedly do disappear in 

 the caudal shifting of the ventral branches, and this process may be 

 of the nature of a degeneration which may liberate a dilute and slowly 

 acting toxic substance comparable to such substances as stimulate 

 the production of endothelial leucocytes in certain pathologic condi- 

 tions, e. g., the relatively slightly virulent toxins arising from typhoid 

 and tubercle bacilli. Then this effect should either be felt throughout 

 the whole of the abdominal aorta, or there should be a progressively 

 decreasing effect, as indicated by the abundance of the desquamating 

 endothelial elements from the distal to the proximal (aortic) portion 

 of these ventral rami. But neither of these conditions obtain. In 

 the older embryos the clusters and desquamating cells are practically 

 limited to the ventral wall ; in the younger embryos (mongoose) where 

 there are fewer atrophying ventral rami, desquamating cells can be 

 found in the lateral and even the dorsal wall, and the ventral cell- 

 clusters are small. The younger the embryo the less differentiated 

 the endothelium. Moreover, the clusters of the older embryos are 

 near the mouths of the vessels, or farthest removed from the site of 

 presumed intensest degeneration; and numerous vessels contain no 

 clusters at all. 



In the 7-mm. mongoose embryo several occluded ventral rami 

 appear, comparable to EmmePs figure 7. The lumen of the vessel is 



