_a._ Arteries of yolk sac (red). _al._ Alimentary cavity. _alv._ Alimentary
vesicle at the posterior end of the alimentary canal. _an._ Point where
anus will appear. _auv._ Auditory vesicle. _bl._ Blastoderm. _ch._
Notochord. _es._ Embryo-swelling. _h._ Head. _ht._ Heart. _m._ Mouth. _mg._
Medullary groove. _mp._ Muscle-plate or protovertebra. _op._ Eye. _sc._
Segmentation cavity. _sos._ Somatic stalk. _ts._ Tail-swelling. _v._ Veins
of yolk sac (blue). _vc._ Visceral cleft. I. _vc._ 1st visceral cleft. _x._
Portion of blastoderm outside the arterial circle in which no blood-vessels
are present. _yk._ Yolk.