On Some Features of Early Embryonic Development Stages of Cnidaria

Division of the life cycle of Cnidaria (except for Anthozoa) into two independent generations, polypoid and medusoid, i.e., metagenesis, is considered to be unjustified. Like other Metazoa, their life cycle can be divided into three periods: embryonic, postembryonic, and definitive, i.e., according to the age [9, 10]. An important feature of Cnidaria is the transition of some postembryonic stages to the sedentary mode of life. As in other animals, this change results in a substantial reduction in organismic integrity and an anarchical type of cell division. Some researchers [3, 5, 7] regard this type of cell division as original. However, the anarchical type of cell division itself is secondary and, for this reason, cannot be ancestral to other types. The statement that the spiral type of cell division originated from the pseudo-spiral type also arouses serious criticism. The spiral type is caused by a change in the structure of the blastomere ooplasm rather than the appropriate arrangement of blastomeres.