STAGE 41 Age 3 d., 4 h.; length 6.7-7.5 mm.

Ext. Crit.: Gills broader and flatter, more laterally directed. *Formation of a left-rostral and a right-caudal furrow in yolk mass; torsion of inter-jacent part of about 45° (1); formation of conical proctodeum*, forming angle of avout 60° with tail myotomes. *Fromation of fin rostral to proctodeum*; verntral outline of yolk makk and proctodeum a smooth concave line.

Int. Crit.: Yolk consumed in skin and brain structures. Lateral infundibular recesses and optic ventricles formed; pia mater discernable at level of mesecephalon. First motor neurons enlarging at edge of gray matter in tail region of spinal cord. Lens cavity disappeared. *Crista posterior split off from medio-caudal sensorial anlage of ear vesicle*; first indications of formation of semicircular canals.

*Several parts of cranum and visceral skeleton procartilaginous*; fen. basicranialis enclosed by procartilaginous structures. Segregation of m. tentaculi; anlagen of m. dilatator laryngis and mm. constrictores laryngis recognizable; further individuation of visceral muscles; eye muscles differentiating. Spiral valve present in conus arteriosus; other i indicated; *ductus Botalli and 1^st aortic arch absent*; coeliaco-mesenteric artery formed; *right omphalomesenteric vein has lost connection with heart and has broken into capillaries in liver*. Beginning of segregation of primordial germ cells from dorsal entoderm.

*Appearance of filter process anlagen along caudal margin of 3^rd visceral arch*. Main gastric glands forming; *oblique right-caudal fold in surface of intestine, corresponding with future tip of helix*; postanal gut disappeared.

(1) The torsion of the intestine is indicated by diagrams added to the plates of stage 41 to 45.