STAGE 50 Age +/- 15 d.; length 20-27 mm.

Ext. Crit.: *Forelimb bud somewhat oval-shaped in dorsal aspect*. *Hindlimb bud longer than

broad, constricted at base, distal outline somewhat conical*.

Int. Crit.: Large number of unicellular glands developing in larval skin. Olfactory bulbs beginning to fuse; *paraphysis beginning to ramify; "Stirnorgan" shifted to position dorsal to paraphysis*; segregation of tectal and tegmental parts of mesencephalon. N. abducens has reached m. rectus posterior. Rohon-Beard cells beginning to decline; *cells of lateral motor columns beginning to differentiate at lumbar levels of spinal cord*. *Brachial ganglia with a few large cells*. *Forelimb and hindlimb nerve entering limb anlagen*; sympathetic chain ganglia formed in trunk region. *Choanae perforated*. *Utriculus divided into pars anterior and pars posterior; ductus perilymphaticus and recessus perilymphaticus papillae amphibiorum formed*.

*Formation of canalis semicircularis posterior of ear capsule; separation of for. endolymphaticum and acusticum*; proc. ventrolateralis palatoquadrati chondrified. Beginning

of differentitation of mesenchyme in hindlimb bud; pair pelvic girdle anlage indicated. Brachial

vein and vena cutanea magna being formed; ischiadic vein clearly distinguishable. *About 20 pairs of glomeruli, and open nephrostomes in mesonephros*. Complete gonadal rudiments formed.

Outpocketings of lungs protruding into dorsal coelomic sacs behind ear capsules. Beginning of

colloid formation in thyroid follicles. Diffuse follicles formed in ultimobranchial bodies.