DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 157 



are brilliantly red in color and their shape and size are apparently 

 normal. 



The blood corpuscles are thus found to differentiate in a typical 

 fashion and to retain their haemoglobin reaction for a long period 

 without having circulated in the vessels. The function of an erythro- 

 cyte would thus seem to be entirely independent of its circulation so 

 far as its capacity to form oxyhaemoglobin goes. These cells are also 

 able to accumulate oxygen within the intermediate cell mass in its 

 central position in the embryonic body. The embryo from which 

 figure 35 was taken had lived 14 days without its blood having 

 circulated, which is about the period required for the young 

 fish to hatch and become free swimming. 



In older embryos the erythrocytes begin to degenerate and in 

 many they lose their red color, the haemoglobin probably break- 

 ing down. The islands on the yolk then become pale in color 

 and finally almost white, as if the cells were dead. The color 

 of the blood cells seems to fade within the embryo earlier than 

 on the yolk-sac as a rule, probably due to the better chances of 

 obtaining oxygen on the thin yolk-sac than in the thicker embry- 

 onic body. The non-circulating specimens often continue to 

 live for a long time even after the blood cells have lost their 

 color. Some such specimens may exist for more than 40 days, 

 which is a very long time considering that the normal embryo 

 may hatch when from 11 to 20 days old. The specimens without 

 a circulation are always weak and delayed in development and 

 of course never succeed in hatching from the egg membrane. 



In a study of the embryos treated with weak alcohol solutions, 

 one very frequently finds cases in which the circulation of the 

 blood may start almost normally and finally stop permanently, 

 although the embryo continues to live. Other embryos may 

 fail to establish a circulation at the proper time and yet may 

 develop a freely flowing circulation of their blood some days later. 

 Again, an embryo may have a fairly normal circulation and for 

 some reason lose it for a few hours, or for one or two days, and 

 then regain it, at first slowly and finally in a fairly strong fashion. 

 All three of these phenomena have also been observed in eggs 

 developing in ordinary sea-water when they were not properly 



