Radioactive labelling method has been effectively applied on the chick blastoderm. The method includes labelling one embryo (donor) and grafting a part of it in similar position on another unlabelled embryo of similar stage (Host). An explanted blastodermis immersed in a medium consisting of the radioactive mtiated (H3) thymidine. In 3 to eight hours the tritiated thymidine is incorporated into the chromosomal DNA of dividing blastodermal cells. The embryo labelled in such type of a way by tritiated thymidine serves as a donor. Another embryo at similar stage of development as attained through the labelled donor in the meantime is then chosen to serve as the host. A small area of the host embryo is excised and replaced by a corresponding piece from the donor of which the fate is to be determined. Healing generally takes place quickly and the development is not impaired if the operation has been done carefully.
The thymidine does not pass out of the nuclei of the labelled cells but stays in the chromosomes of their descendents. Even though the radioactive thymidine present in the DNA is gradually diluted with each subsequent chromosomal replication radioactivity remains for a considerable time. Such type of composite embryo (partly from donor and partly from the host) is tested at a later stage of development for radioactivity through special techniques like autoradiography etc. Only the part(s) or structure(s) developed from the grafted piece display the presence of radioactivity thus establishing the fate of specific area taken from the donor.