STAGE 33/34. Age 1 d., 20 ½ h.; length 4.7-5.3 mm.

Ext. Crit.: Stomodeal invagination a shallow vertical groove. *Dorsal part of eye more pigmented than the ventral part*; distinct melanophores in dorsal part. *Melanophores appearing* dorsally on the head and laterally in a row extending from just below the pronephros backwards. *Length of tail bud about twice its breadth*. Beginning of heart beat.

Int. Crit.: First chromatophores differentiating in head and trunk regions.Appearance of supra- and infraorbital and trunk lateral line placodes. *First optic fibers in middle of chiasmatic ridge and along optic stalks*; appearance of n. oculomotorius. *Appearance of Rohon-Beard cells at trunk levels of spinal cord*. *Nearly spherical lens anlage detached from ectoderm*; distal lens epithelium being formed. Sensorial anlagen of auditory vesicle indicated.

Anlage of m. interhyoideus segregated. *About 32 post-otic somites formed*. Dorsal mesocardium completed and heart twisted; 3^rd aortic arch completed; paired dorsal aorta reaching to pronephros; omphalomesenteric veins opening into functional ducts of Cuvier; anterior cardinal vein formed. Appearance of anlagen of anterior pair of lymph hearts. *3^rd nephrostome completed*; lumen of entire collecting tube of pronephros.

*Thyroid anlage clearly discernable*. Anterior wall of liver rudiment bulging forward.