Fig. 4. The VCAM1-negative proximal region is approximately 220 µm long. The average total lengths (in µm) of allantoises and the VCAM1-negative proximal region from fixed ex vivo conceptuses were plotted as a function of developmental stage of the embryo. The number of allantoises examined at each time point is provided in the base of each bar; vertical lines at the top of each bar represent the s.e.m. Bars with only one specimen (9-s, 11-s) nonetheless contain errors, as measurements were the average of three histological sections for each allantois (see Materials and methods). After 6-somite pairs, the length of the VCAM1-negative region only is indicated because representation of the complete profile of fused allantoises was difficult to achieve in histological sections.