PLATE 2 



EXPLANATION OF FIGURES 



Region of the vascular endothelium at the entrance to an aortic ramus (ar). 

 Is of interest as showing cytological changes in the endothelial cells (c:, 62, ^3) 

 suggestive of an initial stage in the formation of a cell cluster. The drawing 

 includes only the caudal side of the orifice of the smaller vessel, the entrance to 

 which is indicated by the arrow at the right. 



!) Illustrates the cytological structure of an aortic cluster, the discontinuity 

 of the aortic endothelium at its base, evidence of mitotic cell multiplication (d), 

 the gradation in structural characteristics from the basal (eO to the more periph- 

 eral cells of the cluster (^2, Wi) and the apparent detachment of some of the 

 latter as free cells (?H2). The anatomical relations appear brought out to ad- 

 vantage in this particular case, through the artificial separation of the endo- 

 thelial surface (e) from the underlying mesenchyma (s) during the histological 

 preparation of the material. 



10a and 10b Are respectively from directly opposite regions of the ventral 

 and dorsal walls of the aorta and illustrate striking differences in endothelial 

 structure. Note the flattened form and much wider separation of the endothelial 

 cells in 10b as contrasted with 10a and, in the latter case, the kidney shaped nuclei 

 and rounded cells (ei, 62) raised above the general level of the vascular surface. 



11 Transverse section of a ventral arterial branch of the aorta filled with 

 basophilic cells. Some of the cells are phagocytically active (in). A definite 

 lining endothelium is no longer evident, (cf. fig. 7 and fig. 1, da.) 



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