Expression of TH components during the second wave. In situ hybridizations were performed on flat mounted retinas at three different stages of chick development (Stage 29 [A, D], Stage 31 [B, E] and Stage 33 [C, F]. The probes used were Dio2 (A-C) and Dio3 (D-F). The dorsal side of the retina is at the top of the flatmount and the ventral side at the bottom, while anterior (nasal) is toward the left and posterior (temporal) is toward the right. Arrows in (A) indicate the spot that is devoid of Dio2 staining. The arrows in (D) identify the stripe pattern of Dio3 expression at Stage 29.