The Development of Somites in the Chick Embryo.

1. The 'somite centres*. Spratt (1955) put forward the idea that two 'somite centres' exist in the chick blastoderm and that these actively induce the formation of somites. His evidence is based mainly on a series of transection experiments. 2. HenserCs node. This theory is based on the results of experiments in which the node is damaged or extirpated (e.g., Peebles, 1898; Wetzel, 1929; Fraser, 1954). The literature already contains evidence against this theory. 3. The neural tissue. This theory, which has been put forward by various authors (e.g. Griinwald, 1936; Fraser, 1960), is based on the fact that neural tissue and somites are usually closely associated.