ORIGIN OF VASCULAR TISSUES 93 



axial region of the embryonic body. Now it generally happens 

 that conditions which will produce oedematous pericardia will 

 also produce embryos with yolk-sacs whose anterior portions 

 lack endothelium and blood-cells. This fact probably finds its 

 explanation in the circumstance that conditions which will pre- 

 vent circulation will also vitiate these migrating mesenchyme 

 cells to the extent that they are unable to reach the anterior 

 yolk, or are rendered unable to form these vascular tissues even 

 though they reach that location. But here again it is possible 

 to find many exceptional conditions. Many instances can be 

 found in which migrating mesenchyme cells can reach the an- 

 terior yolk and there form blood-cells, even though the heart 

 has never produced any circulation whatever. Thus it is that 

 in chemical treatment, especially if the chemical treatment be 

 severe, a large number of embryos are able to develop red blood 

 cells in only those positions where Stockard claims they can de- 

 velop. There is most often a coincidence between the absence 

 of erythrocytes in the anterior region of the yolk and the anterior 

 axial region. There are, however, many exceptions in which 

 this barrenness of erythrocytes holds good for only one of these 

 regions, even though the blood has not circulated. If an ob- 

 server, in selecting his material, uses as his sole criterion for the 

 failure of circulation the absence of erythrocytes in these regions, 

 his method is capable of giving only one result. 



Examination of the rather small percentage of embryos devoid 

 of circulation which have been able to develop relatively normal 

 eyes and forebrains often reveals in such embryos a tendency to 

 exhibit locally formed and sometimes bilaterally symmetrical 

 patches of erythrocytes at an early stage in the ontogeny. These 

 patches may be in any position with reference to the eyes, and 

 sometimes even in the optic cups. The time during which they 

 exhibit their red color is usually very short. Sometimes following 

 the appearance of a patch of erythrocytes on one side of the head 

 a similar one would appear on the other side symmetrical with 

 the one first observed, the latter having in the meantime lost 

 its color. During the study of such conditions, all possible ef- 

 forts were made to detect any movement of red blood cells. 



