New experimental evidence for somite resegmentation

According to the concept of resegmentation, the boundaries of vertebrae are shifted one half a seg ment compared with somite boundaries. This theory h as been experimentally confirmed by interspecific transplantations of singl e somites. Due to the difficulty of exactly orientating individual somites in the host embryo, the outcome and interpretations of these experiments ha ve occasionally been questioned. This is especially true for the formation of neural arches, their processes, and the ribs. We reinvestigated the form ation of vertebrae in the avian embryo by grafting one and one half somites from quail to chick embryos. This method eliminates the possibility of a w rong somite orientation in the host embryo. Results show that the vertebral body, the neural arch and its processes are made up of material of two adj acent somites. This is also true for the rib, with the exception of the cos tal head, which is formed by only one somite. Whereas in the proximal part of the costal body the chick and quail cell regions border on each other in the middle of the rib, in its distal part quail cells gradually begin to m ix with chick cells. The intersegmental muscles and their skeletal attachme nts sites are formed from the same somite. These results support and comple te the data of previous studies and confirm the resegmentation concept.