PLATE 1 



EXPLAXATIOX OF FIGURES 



1 From a section of a chick embryo of 108 hours, 6.75 mm. (series no. 

 371; slide II, section 80). Outlines drawn under Edinger projection apparatus, 

 X 1500; reduced to XlOOO. In the upper left corner is a portion of the aorta with 

 a few erythrocytes. Outside of the aortic wall is an area of mesenchyme in which 

 some elements show stages of differentiation leading toward large basophilic cells, 

 two of the latter lying free in the tissue spaces at the lower left corner of the 

 figure. These are apparently identical with the developing blood cells (lympho- 

 cytes) in the area vasculosa of the blastoderm as described by Dantschakoff. 



2 From the same embryo as figvu-e 1 (slide II, section 97). Outlines drawn 

 under Edinger i^rojection apparatus, X 1500; reduced to X 1000. A small part of 

 the aorta is shown in the upper right corner. Some of the protoplasmic elements 

 of the mesenchymal syncytium are shown in various stages of differentiation lead- 

 ing toward the free basophilic cells (lymphocytes of Dantschakoff). In the lower 

 left corner is seen one of these cells in the anaphase of mitosis. At the right of 

 this is a practically mature erythrocyte lying free in a mesenchymal intercellular 

 space. 



3 From a section of a chick embryo of 132 hours, 9 mm. (series no. 411; 

 slide IV, section 16). Outlines drawn under Edinger projection apparatus, X 

 1500; reduced to X 1000. This figure, taken just ventral to the aorta, shows a 

 number of the rounded basophilic cells lying free in the mesenchymal intercel- 

 lular spaces. 



164 



