A classification of the stages of embryonic development was determined, based on the morphological characteristics of the yolk cells, epidermis, and the development of the digestive and nervous systems.
Two types of yolk cells are distinguishable, both larger than egg cells.
All the yolk cells, which were placed around the embryo in the previous stage, are absorbed into the embryo.
2006), the second type of yolk cells of Schmidtea polychroa (Schmidt, 1861), the so-called fusiform yolk cells, differentiates from spherical yolk cells.
tigrina), formation of a embryonic gut cavity which contains the yolk cells engulfed by the embryonic pharynx (stages III of P.