PLATE 3 



EXPLANATION OF FIGURES 



4 Photomicrograph of allantoic duct. The lining epithelium is thrown into 

 folds. In the troughs of the folds the epithelium consists of a single layer of 

 cuboidal or flattened cells ; over the crests, of from three to four layers, consti- 

 tuting a stratified columnar epithelium. Art., arteriole; B. I ., 'blood-island' of 

 erythroblastic X 235. 



5 Photomicrograph of pair of blood vessels (the branching venule cut 

 obliquely) and the surrounding mesenchyme (region A.V. of fig. 1). X 300. 



22 



