ORIGIN OF VASCULAR TISSUES 103 



venting any elusive sort of circulation which might take place 

 between the intervals of time at which any single investigator 

 could reasonably be expected to examine this embryo. If the 

 heart could be severed, such a circulation would be precluded. 

 A very fine needle was used to sever the heart at its middle por- 

 tion. The cut ends fused laterally, the fusion being intimate in 

 some places and loose in others. In figure 66 the solid distal 

 portion of the venous end of the heart is shown ventral to the 

 head. On the right side of a section of the heart of this same 

 embryo (fig. 65) taken more posteriorly there will be seen a con- 

 tinuation of the venous portion seen in figure 66; this right por- 

 tion, followed posteriorly, disappears abruptly. The left por- 

 tion of this same section pursued posteriorly is found to be the 

 arterial end of the heart which enters the embryonic body; but 

 if the left portion of the heart of figure 65 be traced anteriorly it 

 disappears abruptly, as did the venous portion when traced pos- 

 teriorly. The two portions are everywhere in the region of fusion 

 well separated mesially. 



Figure 71 is a section taken slightly posterior to that of figure 

 65, near the plane in which the venous portion ends abruptly. 

 In the venous side there are seen to be numerous erythrocytes ap- 

 parently in all stages of differentiation. Some of them have 

 large nuclei and stained intensely red; others with medium- 

 sized nuclei stain purple; still other rounded cells with small 

 nuclei stain blue. It will be noticed that there is no endothe- 

 lium and also no myocardium in this portion of the heart, whereas 

 erythrocytes are plentiful. On the left side, no erythrocytes are 

 seen in section; a few scattered erythrocytes can, however, be 

 found in this arterial portion nearer to the embryonic body. It 

 will be noticed that the endocardium in this portion of the heart 

 exliibits a cuboidal form very unlike the normal squamous epi- 

 thelium. Conditions such as these are often found in these 

 chemically treated embryos. Undoubtedly this is an abnor- 

 mal condition, but it must have some significance. 



So far no attempt has been made to give the source of the eryth- 

 rocytes found in the heart. The conditions in the heart of the 

 embryo from which figure 55 is taken may be of interest as af- 



