cell divisions continue well past the seventh cleavage, as shown by the formation of round blastomeres with less cellular contact with the yolk.
Embryological development of Janolus fuscus follows the patterns characteristic of opisthobranchs and other invertebrates that undergo holoblastic
, spiral cleavage (i.e., nemerteans, annelids, and other molluscs) (Wilson, 1892; Casteel, 1904; Thompson, 1958).
The transition from holoblastic
to superficial cleavage was observed in stage 5 of gammarid embryo- genesis (Scholtz and Wolff, 2002; Browne et al., 2005).
The zygote then undergoes radial, holoblastic
cleavage and forms a hollow coeloblastula.
Cleavage is holoblastic
, equal, and radial [ILLUSTRATION FOR FIGURES 2C-E OMITTED].
Cleavage in hydra embryos is holoblastic
and unipolar; that is, the cleavage furrow progresses inward from one side of each cell.
The main characteristics of this development are (1) incomplete cleavage that becomes holoblastic
; (2) external migration of mesenchyme cells, in the perivitelline space, from the animal to the vegetal pole during gastrulation; (3) hatching that occurs at the end of the gastrulation; (4) differentiation of the vestibule as soon as the end of gastrulation is attained; and (5) production of a juvenile directly from the gastrula without any larval stage.
Although there is no evidence for the occurrence of a conical rotation in the medaka egg, embryos of some primitive fishes that undergo holoblastic
cleavage (species in the Superorder Chondrostei within the Actinopterygii, for example the sturgeon, Acipenser guldenstadti) do form a gray crescent and thus perhaps do undergo a cortical rotation (reviewed by Bolker, 1993; see also Clavert, 1962; Ginsburg and Dettlaff, 1991).
retortiformis eggs, early cell divisions were holoblastic
, and gastrulation was similar to that in indirect-developing embryos containing less yolk.
All the cleavages were of the radial holoblastic