Fig 1. Semithin micrographs showing the distribution of cells within the various layers of the developing CAM at E11 and E18.

a and b: Both the chorion (white arrows) and the allantois (black arrows) are relatively thin early during development and there are numerous cells concentrated in the subepithelial layer of the chorion (white arrowheads) and also in the subepthelial layer of the allantois (black arrowheads). Note both the chorion and the allantois are of irregular thickness. In the middle part of the mesenchymal layer (asterisks), cells are few. c and d: At E18 the CAM has reached maturity and the chorion (white arrows) and the allantois (black arrows) are both 3–4 times thicker, have an almost uniform thickness and their basal aspects are delineated by a prominent basal membrane (white and black arrowheads respectively). Distribution of cells in the mesenchymal layer (asterisk) is almost uniform.