STAGE 28. Age 1 d., 8 ½ h.; length 3.8-4.0 mm.

Ext. Crit.: Fin extending up to anus. *Fin broadest and distinctly divided into outer transparent and inner translucent band*.

Int. Crit.: Beginning of secretion of cement gland. Hypophyseal anlage penetrated beyond optic stalks; infundibulum rather well developed, ventral wall thinning out; *first fibres (white matter) developing along ventro-lateral portion of mes- and rhombencephalon and along varying portion of anterior half of spinal cord*. First epibranchial placodes segregated. *Marginal invagination of retinal layer extended around eye vesicle, central portion still convex*. *Ear vesicle detached from epidermis*.
*20-22 Somites segregated*. *First formation of endocardial tube and beginning of formation of pericardial cavity*. *1^st nephrostome funnel clearly indicated*, the following two just distinguishable; coelomic filter chamber indicated.