STAGE 42 Age 3 d., 8 h.; length 7.0-7.7 mm.

Ext. Crit.: Beginning of formation of opercular folds. *Torsion of intestine about 90°; proctodeum connected with yolk mass by short horizontal intestinal tube*.

Int. Crit.: First appearance of dura-endocranial membane at level of mesencephalon; anlage of "Stirnorgan" beginning to segregate from epiphyseal anlage; *first fibers in comm. cerebellaris*. N. opthalmicus profundus v and n. maxillomandibularis separated by proc. ascendens palatoquadrati; n. glossopharyngeus innervating m. levator arcuum branchialium. *Rods and cones distinguishable in pars optica retinae*.

Beginning of chondrification of visceral skeleton and palatoquadratum. *Head somites I and II disappeared*; left and right anlagen of mm. constrictores laryngis meeting dorsally and ventrally. Yolk consumed in tail structures. Right omphalo-mesenteric vein receiving main portal vein; abdominal vein present. First appearance of interrenal cells.

*5^th visceral pouch perforated; filter process anlagen appearing along caudal margin of 4^th and 5^th visceral arches*.