172 CHARLES R. STOCKARD 



changed into any type of blood cell. The brown chromatophore 

 in early stages may accidentally reach the blood current; it then 

 becomes spherical and may be readily observed for a long time 

 on account of its huge size as compared with the blood cells. It 

 never, however, changes in type. 



In specimens without a circulation of the blood both types of 

 chromatophores arise in a normal manner and differentiate 

 normally. Their arrangement along the vessel walls fails to 

 occur and they remain scattered over the yolk or collected about 

 the plasma filled spaces. The heart in such embryos is sheathed 

 with pigment, while the normal heart never has a chromatophore 

 on it. 



3. The elongate spindle cells with their delicate filamentous 

 processes are small in comparison with the two chromatophore 

 types. These spindle cells retain in general their original ap- 

 pearance, but their behavior is most important. In embryos 

 of about 48 hours such cells aggregate into certain rather definite 

 groups; later, these groups become more linear in shape and finally 

 these hnes of cells arrange themselves so as to form tubular 

 vessels. Several of the larger vessels arise independently upon 

 the yolk, and certain ones of them later become connected with 

 the venous end of the heart, while in all cases capillary nets which 

 also arise independently become connected with the larger ves- 

 sels. These processes may actually be followed through every 

 step in the living yolk-sac. 



The wall of the early vessels is very irregular with spaces exist- 

 ing between the component cells. Corpuscles are often caught in 

 these spaces or entangled in the filamentous processes of the 

 endothelial cells. Such conditions in sections would appear as 

 though the corpuscles actually formed a part of the endothelial 

 wall and might incorrectly be interpreted as endothelial cells 

 changing into blood cells. Nothing has been seen in the living 

 embryos to indicate that an endothelial cell has the power to 

 produce a blood cell or to change into a blood cell of any type, 

 but much has been seen to the contrary. 



The generalization particularly made by Thoma ('93) that 

 larger vessels arise from a net-work of capillaries is not true for 

 the large vitelhne vessels on the fish yolk-sac. It is also found 



