98 FRANKLIN P. REAGAN 



failure of an inhibiting nerve-supply to its musculature. Un- 

 fortunately; I was unable to section this embryo, as it was in some 

 way mislaid. This misfortune was in a measure compensated 

 for by the fact that the observations on this embryo were made 

 under circumstances (The Marine Biological Laboratory, Woods 

 Hole, INIassachusetts) in which I could avail myself of the coun- 

 sel of many investigators, a number of whom were so kind as to 

 examine the embrj^o in question and verify my observations. 

 At the time at which the sketch for figure 69 was made, the em- 

 bryo was examined by Profs. E. G. Conklin, H. McE. Knower, 

 E. V. Cowdry, E. I. Werber and many others. At a time some- 

 what later after the yolk-sac circulation had been established, 

 the embryo was examined by Prof. C. R. Stockard. This em- 

 bryo furnishes an example of a completely isolated functional 

 heart containing blood-cells. 



Attention is now called to the conditions in one of the most 

 important embryos obtained from the entire study. In this for- 

 tunate case the conditions which were able absolutely to pre- 

 vent the formation of vascular tissues on the anterior yolk, and 

 to cause the venous end of the heart to be solid, did not prevent 

 the formation of ery^throcytes in the anterior vessels or in the 

 anterior mesenchyme. This embryo is also to be considered pre- 

 sently in connection with the formation of blood in the aorta and 

 the liver. It seems certain that mesenchyTne cells in these vari- 

 ous parts were locally transformed into erythrocytes. On the 

 left side of figures 62 and 63 wdll be noticed a cup-like invagina- 

 tion of the fore-brain tissue resembling closely an optic cup. In 

 the place in which one should expect to find a lens, there devel- 

 oped at an early stage a conspicuously red blood-lacuna. No 

 other endothelium or erythrocytes could be found in connection 

 with this lacuna, either in the living or sectioned material. Dor- 

 sal to the optic tissue on the right side of figure 55 are a few 

 erythrocytes which were found lying in the crevices between the 

 peculiar folds of nervous tissue. Some of the cells in these loca- 

 tions seemed to be transforming into erythrocytes, but bore un- 

 mistakable traces of a mesenchymal nature. Such isolated cells 

 were found entirely out of relation to all endothelium. 



