Fig. 4. Amphi-hairyA, Amphi-hairyB, Amphi-hairyC and Amphi-hairyD expression in amphioxus neurulae by whole-mount in situ hybridisation. Lateral views (A-D,H,J,N,R,V) have anterior towards the left and dorsal upwards. In dorsal views (I and O), anterior is towards the left. In transverse sections (E-G,K-M,P,Q,S,T,W,X), dorsal is upwards looking from a posterior direction. (A) Amphi-hairyA expression in the endoderm and the posterior dorsal nerve cord (arrow) of an early amphioxus neurula. (B) Amphi-hairyA expression in the endoderm and the posterior dorsal nerve cord (arrow) of a late neurula. The expression in the Hatschek's left diverticulum is shown by arrowheads in A and B. (C) Oblique view of an early neurula that exhibits Amphi-hairyB expression in the whole neural plate and the posterior compartment of the segmented somites (the last somitic border is marked by an arrow). (D) Lateral view of an early neurula showing Amphi-hairyB expression in the neural tube, the posterior PSM and the segmented somites. (E) Cross-section through level i in D showing the expression of Amphi-hairyB in the neural tube (asterisk). (F) Cross-section through level ii in D showing the expression of Amphi-hairyB in the neural tube (asterisk) and the segmented somites (arrows). (G) Cross-section through level iii in D showing the expression of Amphi-hairyB in the posterior PSM (arrows). (H) Lateral view of late neurula showing Amphi-hairyB expression in the neural tube and the posterior PSM (arrow). (I) Dorsal view of the specimen in H; the expression in the PSM is shown by arrows. The gap in the anterior neural tube domain is shown by a line in H and I. (J) Lateral view of an early neurula stained for the Amphi-hairyC mRNA with the expression in the neural plate, the somites and the posterior PSM. (K) Cross-section through level i in J showing the expression of Amphi-hairyC in the neural plate (arrowhead) and the anterior gut (arrows). (L) Cross-section through level ii in J showing the expression of Amphi-hairyC in the neural tube (arrowhead) and the somites (arrows). (M) Cross-section through level iii in J showing the expression of Amphi-hairyC in the posterior neural plate (arrowhead) and mesoderm. The low expression levels in the forming notochord is shown by an asterisk. (N) Lateral view of a late neurula stained for the Amphi-hairyC mRNA showing the expression in the neural plate, the anterior gut, the somites and the posterior PSM. (O) Dorsal view of the specimen shown in N. The arrows indicate the expression in the PSM. (P) Cross-section through level i in N showing the expression of Amphi-hairyC in the neural tube (arrowhead) and the segmented somites (arrows). The low expression in the dorsal notochord is shown by an asterisk. (Q) Cross-section through level ii in N showing the expression of Amphi-hairyC in the posterior mesoderm and no longer in the neural plate (asterisk). (R) Lateral view of an early neurula showing Amphi-hairyD expression in the anterior endoderm, the neural tube and the notochord. (S) Cross-section through level i in R showing the expression of Amphi-hairyD in the neural plate (arrowhead) and the gut (arrow and double arrow). (T) Cross-section through level ii in R showing the expression of Amphi-hairyD in the posterior mesoderm (arrow). The lack of Amphi-hairyD expression in the posterior neural plate is shown by an asterisk. (V) Lateral view of a late neurula showing Amphi-hairyD expression in left gut diverticulum (arrow), the neural tube and the notochord. (W) Cross-section through level i in V showing the expression of Amphi-hairyD in the neural plate (arrowhead), the ventral notochord (asterisk) and the gut (arrow). (X) Cross-section through level ii in V showing the expression of Amphi-hairyD in the posterior mesoderm. The asterisk marks the expression in the forming notochord.