A-H. Somites (in dorsal view) electroporated in the DML at E2.5. Nuclei number per fiber was analyzed at E5.5. Doxycycline induction of constructs was performed at E3.5. E,H. Immunostaining with anti HA tag antibody (Rac1T17N and Cdc42 T17N are both HA tagged) to ensure that doxycycline induction was efficient. I. Quantification of nuclei number per fiber at E5.5. Whiskers indicate minimal and maximal values; the bottom and top of the box indicate the first and third quartile; the horizontal line indicates the median; and ‘+’ indicates the mean for each set of values. J. Quantification of fiber length compared to nuclei number. Control: R2 = 0.007, "Y = 4.087*X + 404.9"; Rac1T17N: R2 = 0.016, "Y = 23.54*X + 378.3"; Cdc42T17N: R2 = 0.035, "Y = 11.34*X + 368.6". All images are whole-mount confocal stacks of fixed embryos, immunostained for EGFP (green) and RFP (red). Arrowheads (B,D,E,G,H) indicate cell nuclei within selected fibers. ***p<0.0001. Scale bars: 100μm in A,C,F; 50μm in B,D,E,G,H.