ORIGIN OF VASCULAR TISSUES 101 



periments, in fact, preceded my own, but his results were un- 

 known to me until my return from the Woods Hole laboratories. 

 He has kindly placed at my disposal an embryo on which he made 

 daily observation, and which he sketched in the living condition. 

 In the living embryo a group of erythrocytes was observed dor- 

 sal to each eye. A section through that region is shown in fig- 

 ure 76. On the left side the erythrocyte group is the more 

 prominent. It is not surrounded by a distinct endothelium. In 

 sections of this region, such crescent-shaped groups of erythro- 

 cytes over the eyes are not of rare occurrence. They are found 

 sufficiently often to indicate that the mesenchyme of this region 

 may be a true erythrocyte-anlage. On consulting Werber's fig- 

 ure 24 (78) it will be seen that he found a blood-anlage very 

 similarly located. He describes the blood cells in this region as 

 leucocytes ''evidently of polymorphonuclear variety," subscrib- 

 ing at that time to the view of Stockard that the blood cells pro- 

 duced in the anterior region are leucocytes. I regard it doubt- 

 ful that these are really leucocytes. Be this as it may, the ho- 

 mologous region of the mesenchyme which in Werber's figure 

 seemed to give rise to leucocytes gave rise to erythrocytes in the 

 experiment performed by Professor McClure. It will be seen in 

 figure 77 that the arterial portion of the heart is solid; sections 

 show this embryo to be devoid of ventral aorta, aortic arches, 

 and anterior dorsal aorta. Examination of sections somewhat 

 posterior to the eyes reveals a region posterior to the erythro- 

 cytes on both sides of figure 76, which contains no haemal vas- 

 cular tissue whatever (fig. 78). Thus we see that the vascular 

 anlagen anterior to this are completely isolated. A section 

 through one of the nasal pits of this embryo (fig. 79) shows a 

 small vascular cavity containing a few erythrocytes. The endo- 

 thelium of this cavity narrows down to an exceedingly fine tube 

 which can be traced back to the region of the vascular anlage on 

 the same side in figure 77. The tissue in this embiyo is in a re- 

 markable state of preservation. It was stained in Delafield's 

 haematoxylin and orange G. The differentiation of the eryth- 

 rocytes is not very great in this stain, so that they might easily 



