October 15, 1915] 



SCIENCE 



539 



one depositing a black and the other a brown- 

 ish-red pigment. 



The black chromatophore increases rapidly 

 in size and by the end of the third day be- 

 comes an enormous ameboid body wandering 

 over the yolk. These cells are attracted to the 

 walls of blood vessels and plasma-filled spaces, 

 such as the pericardial cavity becomes in indi- 

 viduals without a blood circulation. When the 

 embryo is five days old the chromatophores are 

 abundantly arranged along the walls of the 

 vitelline vessels, but the pigmented cells are 

 distinctly separate. After this time neigh- 

 boring cells begin to fuse along their adjacent 

 borders and large pigment syncytia are formed 

 which completely surround and ensheath the 

 vessels. A single syncytium is often of consid- 

 erable extent. 



The brovm chromatophores have a somewhat 

 different history. They never become so mass- 

 ive as the black, and their processes are more 

 delicate and graceful in appearance. Tet these 

 cells also attain a large size and in embryos of 

 Y2 hours are scattered over the entire yolk- 

 surface. After the third day when the blood 

 begins to flow in the yolk vessels, the brown 

 chromatophores likewise become attracted to 

 the vessel wall. These exquisitely branched 

 cells apply themselves to the wall of the vessel 

 and may often completely surround it. This 

 type of chromatophore, however, always main- 

 tains its cellular individuality and never fuses 

 with other cells to form a syncytium, as is the 

 case with the black type. 



The function of the chromatophores on the 

 yolk-sac is most difficult to decide, but one 

 thing is certain, they never become changed 

 into any type of blood cell. The brown chroma- 

 tophore in early stages may accidentally reach 

 the blood current; it then becomes spherical 

 and may readily be 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 t3rpes of chromatophores arise in a 

 normal manner and differentiate normally. 

 Their arrangement along the vessel walls fails 

 to occur. The chromatophores, therefore, re- 



main 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. 



The elongate spindle cells with their deli- 

 cate filamentous processes are small in com- 

 parison with the two chromatophore types. 

 These spindle cells retain in general their 

 original appearance, but their behavior is most 

 important. In embryos of about forty-eight 

 hours such cells aggregate into certain rather 

 definite groups ; later, the groups become more 

 linear in shape and finally these lines of cells 

 arrange themselves so as to form tubular ves- 

 sels. Several of the larger vessels arise inde- 

 pendently 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 vessels. These proc- 

 esses may actually be followed through every 

 step in the living yolk-sac. 



The wall of the early vessels is very irreg- 

 ular, with spaces existing between the compo- 

 nent cells. Corpuscles are often caught in these 

 spaces or are entangled in the filamentous proc- 

 esses of the endothelial cells. Such conditions 

 in sections would appear as though the cor- 

 puscles actually formed a part of the endothe- 

 lial 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 strikingly made by 

 Thoma^ that larger vessels arise from a net- 

 work of capillaries is not true for the large 

 vitelline vessels of the fish yolk-sac. In the 

 specimens without a circulation of the blood 

 the vessels arise and increase in size and per- 

 sist for a long time without ever experiencing 

 any effect of the blood current upon their walls. 



2 Thoma, E., ' ' Untersuehungen ueber die Histo- 

 genese und Histomeehanik des Gef asssystems, " 

 Stuttgart, 1893, and "Text-Book of General 

 Pathology and Pathological Anatomy, ' ' trans, by 

 Bruce, London, 1896. 



