232 



BIOLOGICAL LECTURES. 



Fig. 5. 



vessels which is marked by fine lines is covered by red pigment 

 cells, the other part by black pigment cells. 



In my first publication on the subject I confined myself to 

 a description of the origin of the marking in the yolk sac. I 

 mentioned, however, that in the embryo itself the marking 



seemed to originate in the same way. 

 I have since examined the origin 

 of the marking in the embryo more 

 closely, and have found it the same 

 as in the yolk sac. Fig. 5 shows 

 a comparatively young embryo, in 

 which the pigment cells are still 

 scattered irregularly. Later on, 

 however, the pigment cells here, 

 too, creep upon the blood vessels. 

 Fig. 6 shows the blood vessels and 

 pigment cells in the tail of an older 

 embryo in which the marking has 

 taken place. In some of the blood vessels the direction of the 

 blood flow is indicated by arrows. This drawing, like all the 

 others, was taken from life.^ We see that in the embryo, as in 

 the yolk sac, all the pigment cells have crept upon blood ves- 

 sels. I should like 

 to direct the atten- 

 tion of the reader 

 to one special 

 point : in the mid- 

 dle of the body two 

 large blood vessels, 

 an artery and a 

 vein, run side by 

 side. They are 

 marked by two ar- 

 rows. Here all the chromatophores have crept upon the artery 

 while the vein has remained free. It is obvious that the artery 

 contains more oxygen than the vein, and this seems to suggest 



1 These drawings were made by Mr. Bridgham, of Providence, R. I., under my 



direction. 



Fig. 6. 



