Pattern and Time Course of Cleavages in Early Development of the Ovoviviparous Pond Snail, Sinotaia quadratus historica

The pattern and time course of cleavage during early development of the ovoviviparous pond snail, Sinotaia quadratus historica, in in vitro culture were investigated. The fertilized egg, 25–30 μm in diameter, underwent cleavage by repeated constriction and compaction as in blastocyst formation in mammalian embryos. The cleavage was slightly unequal and dextrally spiral, although a slight time lag in cleavages of blastomeres was observed after the 2nd cleavage. A small polar lobe was formed at the 1st cleavage, but not at the 2nd cleavage. Each cleavage proceeded very slowly under the experimental conditions, the 1st, 2nd, 3rd and 4th cleavages taking 22 hr or more, 9 hr and 10 hr, respectively. The cleavage pattern in in vitro cultures observed by light microscopy was confirmed by scanning electron microscopy.