Time course of somite number during chick somitogenesis. Extension of the somite formation period. (A) Graph plotting increasing time intervals along the x axis and increasing number of somites and the corresponding HH stage of chick development along the y axis. The red line is a theoretical line showing the progression if the speed of somitogenesis were to remain constant at 90 minutes throughout development. Each blue dot corresponds to an individual sample/embryo. The blue line is a best fit line. Dotted green line demarcates the average final number of somites. (B-G) Fix and culture analysis using the half embryo system using the caudal region of a chick embryo. Left half samples were immediately fixed whereas right half samples were cultured in vitro for different periods of time and then the expression of c-Lfng was analysed in the two halves by in situ hybridisation. (B) HH stage 20 and (C) stage 23 samples cultured for 90 minutes. (D-G) HH stage 23 samples were cultured for (D) 60 minutes, (E) 90 minutes, (F) 120 minutes and (G) 150 minutes.