The embryological development of Turbellaria Kalyptorhynchia (Rhabdocoela)

Abstract

The ectolecithal egg mass of most Turbellaria Kalyptorhynchia is formed by two eggs and about hundred vitelline cells. The fertilization of the primary oocyte takes place in the genital atrium and maturation occurs in the uterus during the formation of the egg-shell. After its sclerotinization, the egg-capsule is fixed on a substratum by the secretion of the uterine glands.

The cleavage of the oligolecithal egg is total and unequal. The two blastomeres AB and C appear successively at the anterior pole and on the side of the macromere. The cell multiplication, which almost exclusively occurs in the D-quadrant, gives the morula where some micromeres and two endodermic mother-cells can be distinguished.

The fragmentation of the yolk mass round each of the two embryos is followed by the formation of a perivitelline embryonic epithelium constituted from micromeres migrating through the yolk by means of amoeboid movements. Gastrulation takes place by centrifugal migration of the blastomeres following the yolk absorption by vitellophages of endodermic origin.

Subsequently, the undifferentiated endodermic cells make a temporary embryonic intestine round the yolk-filled vitellophages that are gathered in the body-cavity of the embryo.

During gastrulation a small pharyngeal blastema appears in the ventral part of the embryo. It will form later the whole digestive tract. The other blastomeres gather in a cephalic blastema which will give the nervous system, the subepidermal muscle cells and the proboscis. The definitive epidermis differentiates from the perivitelline embryonic epithelium.