STAGE 44 Age 3 d., 20 h.; length 7.8-8.5 mm.

Ext. Crit.: *Appearance of tentacle rudiments*. Opercular folds protruding further. *Coiling part of intestin showing S-shaped loop; torsion about 360°*. Blood-circulation in gills usually ceased (gills smaller).

Int. Crit.: Caudal lateral line appearing on pre-anal fin. *Beginning of invagination of plexus formations*. First appearance of dura-endocranial membrane around anterior portion of spinal cord; motor neurons standing out in tail region of spinal cord. Beginning of segregation of inner choroid and outer scleral coat of eye. *Ear vesicle divided into pars superior and pars inferior*.

Beginning of chondrification of comm. quadrato-cranialis anterior and arcus subocularis; *plana branchialia chondrified*. *Head somite III disappeared*. "Urwirbelfortsatze" splitting into anlagen of m. obliquus abdominis and m. rectus abdominus. Valves in conus fully developed; circulation in filter apparatus established; *lateral postcardinal veins indicated; posterior vena cava established*. *Aditus laryngis partially formed*; anlagen of filter apparatus developing along cranial margins of visceral arches.