Experimental analysis of control mechanisms in somite segmentation in avian embryos. I. Reduction of material at the blastula stage in Coturnix coturnix japonica.

The blastulae of unincubated eggs of the quail, Coturnix coturnix japonica, have been bisected in ovo, using the technique of Lutz (1949). Some embryos were harvested after 24 h and found to possess two primitive streaks. Most were fixed at 48 h or 72 h. Some were found to have regulated to form almost normal single axes, whilst others had developed into duplicitas anterior embryos, separate twins or collided axes. All three types of twinned embryos were smaller than the control embryos. The number of somites was not however reduced in the shorter embryos. This finding corresponds with a similar result obtained by Cooke (1975) who reported that if a Xenopus blastula is reduced in size, it nevertheless develops the correct number of somites. The quail however adjusts the shape of the individual somites so that they fit into the reduced body length, whereas Xenopus reduces the size of somites. No miniaturized somites were ever seen in these quail embryos. As a result of the present experiments, it was concluded that the length of incubation time does not directly control the rate of somite formation, because different numbers of somites were found in twins which possessed identical genomes and had developed in almost the same environment for identical periods. In addition, the size of the area pellucida does not appear to control somite formation. Probably, the most important influence is the regression of the node.