[Critical periods in the development of spurs in light of the theory of phylembryogenesis].

Two critical periods have been established in the development of spurs representing a derivative of the corneous squama. During the first period the exposure to x-rays results in the development of the corneous squama instead of the spur, and the same exposure during the second critical period completely suppresses the development of the spur. These periods seem to correspond to the truning-points in the chain of morphogenetical processes of the spur formation associated with phylembryogeneses. Department of Embryology, Institute of Experimental Medicine, USSR Acad. Med. Sci., Leningrad.