PLATE 4 



EXPLANATION OF FIGURES 



6 Drawing of blood-island, from region just below the allantoic duct (fig. 1, 

 All.). Peripherally the cells are becoming differentiated into an endothelium. 

 Centrally the syncytial mass is becoming vacuolated through the appearance of 

 intercellular spaces, and certain of the cells have entered the early erythroblast 

 ('megaloblast') stages (a and b). One erythroblast (d) contains a large vacuole. 

 A hemoblast (e) has differentiated an intracellular erythrocyte. A hemoblast 

 (h) is separating from the differentiating endothelium. Between the two endo- 

 thelial cells below, appears another hemoblast. Figures 6 to 9 are magnified 

 1500 diameters. 



7 Blood-vessel in process of differentiation from the mesenchyme. This 

 drawing is from the region to the right of the allantoic duct between the two 

 umbilical arteries (fig. 1), and includes approximately the middle third of this 

 entire vascular anlage. The forming lumen contains seven young erythroblasts 

 ('megaloblasts'), separating out of an originally syncytial mass. 



8 Young vasofactive cell, with generally mesenchymal features and three 

 vacuoles, the precursors of a capillary lumen. 



9 Slightly older vasofactive cell with large vacuole, the underlying nucleus 

 assuming endothelial features. 



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