Stage 25 (2 days 2 hrs) 18-19 somite stage
Drown by T. Iwamatsu
(Abbr. cd: Cuvierian duct, gt: gut tube, l: lens)
Stage 25 (2 days 2 hrs) 18-19 somite stage (Onset of blood circulation):
When blood circulation begins, the spherical blood cells are first pushed out of the blood island (7th-15th somites) toward the vitello-caudal vein. The blood is pumped (70-80 heart- beats/min) from the heart out into the anterior cardinal vein and the dorsal aorta roots. The dorsal aorta branching off the perceptible bulbus arteriosus is paired anteriorly with
continuations extending to the head as the internal carotid arteries. The carotid artery splits to form the optic plexus, which connects with the left and right ducts of Cuvier. The left and right dorsal aorta roots run caudally until they join to form the dorsal aorta. The dorsal aorta is unpaired through the trunk region and continues into the tail as the vitello-caudal artery. A counter current of the blood stream from the heart into the aorta is still observed. Otoliths appear as two conglomerates of small granules lying against the inner surface of each well-expanded otocyst. The embryonic body encircles nearly 7/12 of the yolk sphere. The dogleg-shaped somites form a herringbone pattern between the 3rd and 10th somites. Kupffer's vesicles have disappeared completely. The bulge of the liver anlage appears at the 1st-3rd somites just posterior to the future position of the left pectoral fin in the 19 somite stage.