244 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



liquefaction of the angioblasts in living chick at least half of the cytoplasm has 

 liquefied (fig. 26, plate 6). 



In connection with a specimen such as that shown in figure 6, plate 2, it is 

 interesting to consider at what point in the development of the chick the hemoglobin 

 appears. It is, I think, generally admitted that the first masses of cells, which were 

 called blood-islands by the early embryologists, in the stages of 1, 2 and 3 somites 

 have no hemoglobin at all. In the area pellucida in a living specimen one can dis- 

 tinguish hemoglobin by a slight yellow color under the microscope, although the 

 amount is too small to fix and stain. On the other hand, in the cells of the area 

 opaca, which, in these hanging-drop preparations, must always be seen against a 

 background of endodermal cells packed with yolk, one can not be so sure of detecting 

 the first traces of hemoglobin by this color reaction. In specimens of 6 somites, 

 however, in which there has been no liquefaction of the angioblasts to form blood- 

 vessels in the outer rim of the area opaca, it is certain that one can see no traces of 

 the yellow color in the blastoderm in a hanging-drop preparation. In this con- 

 nection it is interesting to note the work of two Russians on the time of appearance 

 of the hemoglobin. These works I have not seen in the original but know only 

 through a quotation by Maximow (1909, p. 465): 



"So fand Smiechowski (1892) dass das Haemoglobin sich optisch und chemisch erst in 

 Hiihnerembryonen nachweisen lasst, die schon 12 differenzierte Segmente besitzen. Wulf 

 (1897) der das Haemoglobin speziell mittelst des Spektrokops suchte, fand die ersten Spuren 

 erst beim Hiihnerembryo mit 6 Segmenten, wahrend das voile Haemoglobinspectrum erst 

 mit 9 Paar Segmenten erschien." 



The time at which hemoglobin appears, in terms of the number of somites, 

 probably varies within wide limits, but the evidence all tends to indicate that it 

 occurs after the time when the vessels differentiate out of angioblasts, and that 

 hemoglobin is not formed until blood-plasma is produced from the liquefaction of 

 some of the protoplasm of angioblasts. This point seems to me to emphasize again 

 the disadvantage of calling the masses of primitive mesoderm blood-islands. All of 

 this evidence is in harmony with the point of view of Madame Danchakoff, now 

 accepted, that in the chick red blood-cells form only within the lumen of a vessel, 

 and suggests at least that the production of some plasma by the liquefaction of 

 certain cells has something to do with the capacity of the cells in the islands to 

 develop hemoglobin. Recently, Madame Danchakoff (1918) has brought forth 

 evidence of the power of endothelium in the chick to produce red cells, even in 

 zones where the endothelium has been so injured that it does not completely inclose 

 a lumen. These observations are especially interesting in connection with the 

 origin of red cells in mammals. It is generally believed that hemoglobin-bearing 

 cells in mammals develop outside of vessels, which would indicate that the theory 

 concerning the importance of endothelium and of plasma in the production of red 

 cells can not be generally applied. 



According to Maximow (1909), there are two different types of red blood-cells 

 in mammals: "Sehr merkwiirdig ist die Tatsache, dass es beim Saugetier zwei 

 so scharf geschiedene Typen von roten Blutzellen gibt; die primitiven und die 



