STAGE 45 Age 4 d., 2 h.; length 8-10 mm.

Ext. Crit.: Operculum partly covering gills, edge still straight. *Intestine spiralized in ventral aspect, showing 1 ½ revolutions*. Beginning of feeding.

Int. Crit.: Parietal sense organs present. Nasal organs beginning to withdraw from hemispheres. Motor neurons standing out in posterior trunk region of spinal cord. Choroid fissure closed except for passage of hyaloid artery. Outgrowth of parachordalia and formation of hypochordal commissure. Mesenchymal condensations of hindlimb anlagen connected by future anlage of pelvic girdle dorsal to anal tube; first thickening of ectoderm over hindlimb bud area. *Partition wall between atria completed*. Anlage of spleen well defined. *3-4 pairs of mesonephric units discernable in mesonephric anlage*. *Paired genital ridges formed*. Interrenal blastema being formed. "Rakes" on 3^rd visceral arch branched; *anlagen of filter processes on inner surface of operculum*.