PLATE 15 



EXPLANATION OF FIGURES 



64 Section of the same embryo slightly posterior to the optic stalks, showing 

 the precardinals as haemophoric vessels. The erythrocytes are angular in con- 

 tour (especially in the vessel on the right of this figure) due to their crowded 

 condition. A few of the erythrocytes in the less crowded vessel on the left have 

 become rounded. 



65, 66, 67 and 68 Transverse sections of the embryo from which figure 49 was 

 drawn. 



65 Section just posterior to the optic cups passing through the anterior ends 

 of the haematopoetic areas of this region as shown in figure 49. In this figure, 

 as in figure 66, the position of the blood-cells is roughly and arbitrarily indi- 

 cated by heavy black dots. 



66 Section passing through the posterior portions of the optic cups, and 

 showing the position of the median blood anlage of figure 49. The heart in this 

 figure is a solid continuation of the left portion of the double body labeled heart 

 in figure 65. 



67 An accurate detail of the blood-producing areas seen on the right side 

 of the head in figure 65. Conditions in the lower lacuna seem to be farther 

 advanced. Certain mesenchjnne cells (lightly stippled) are in a stage of tran- 

 sition to erythrocytes (cytoplasm clear). A blood space is forming while the 

 mesenchyme cells bounding it seem to be receding to form endothelium. The 

 free erythrocytes are rounded in contour. 



68 An accurate detail of the median dorsal blood anlage of figure 66. The 

 blood-cells are very strongly eosinophilous but are crowded and possess angular 

 contours. 



ABBREVIATIONS 



Erth., erythrocytes ///., heart 



160 



