Fig. 2. Transverse sections of a 9.5 dpc (26 somite) pEEBZ transgenic embryo following X-gal staining. When using the criteria of Spörle and Schughart (Spörle and Schughart, 1997) this embryo is at the 27 somite stage. (PSM) No transgene expression can be seen in the presomitic mesoderm adjacent to the first somite. As soon as the somite is born (So Ip), ß-galactosidase-positive cells are detected adjacent to the dorsal neural tube. (So IIa-IIIa) Stronger epaxial expression is seen in the dermomyotome of the subsequent, older somites. (So V-XIX) The expression of ß-galactosidase is observed more hypaxially throughout the dermomyotome and the myotome as the somite matures (p, posterior; a, anterior).