STAGE 40 Age 2 d., 18 h.; length 6.3-6.8 mm.

Ext. Crit.: *Mouth broken through*. Length of gills about twice their breadth, posterior one sometimes also showing a branch. *Outlines of proctodeum and tail myotomes forming angle of 90°*. Beginning of blood circulation in gills.

Int. Crit.: Supraorbital and occipital lateral line anlagen segregating into sense organs; dorsal and middle lateral lines of trunk extending to base of dorsal fin at base of tail, and segregating proximally into sense organs. *First fibers in comm. habenularis*. Spinal nerves reaching lateral plate in trunk region. Outer and inner plexiform layers forming in pars optica retinae; fasciculus opticus formed; *nucleai of central lens fibers degenerating*; inner corneal layer formed. *Latero-cranial sensorial anlage of ear vesicle spit into medial and lateral (crista externa) anlagen*. The various parts of cranium and visceral skeleton well distinguishable as mesenchymatous condensations. Individualization of mm. levatores mandibulae and m. intermandibularis. Formation of m. genio-hyoideus. *About 45 post-otic somites*. Elastica interna forming at base of tail. *Anlage of forelimb atrium formed dorso-caudal to 5^th visceral furrow*. *Hepatic vein present; left omphalo-mesenteric vein, recieving subintestinal vein, has lost connection with heart and has broken into capillaries in liver*; small external jugular and pulmonary veins formed.

*Oral plate ruptured and 3^rd and 4^th visceral pouches perforated*. 6^th visceral pouch indicated; *tracheal cavity separated from gastro-duodenal cavity*; paired thyroid anlage detached from "thyreoglossal duct"; anlagen of thymus gland and ultimobranchial bodies indicated. Preliminary liver cavity occluded; gall bladder anlage formed; *the three pancreatic rudiments fused*.