Development of Tethya serica Lebwohl, a Tetraxonian Sponge : I. Observations on External Changes
摘要
1. In this report, the external observations of developmental changes after cleavage in a tetraxonian sponge, Tethya serica Lebwohl, are described. 2. The egg segments totally and equally, but there are seen many variations in cleavage patterns, which probably owes to the adhesion of the egg to the substratum and to the presence of heavy yolk. 3. At the surface of the embryo where the embryo adheres to the substratum, there develops from marginal cells of the embryo a number of thread-like processes. These processes help to fix the embryo more firmly to the substratum, but soon degenerate after the appearance of the multicellular projections, which are the rudiment of the root. 4. The multicellular projections come together under the growing embryo, and are united into one at their base, thus becoming the rudiment of the root. Distal ends of the projections which remains disunited, are often furcated, and constantly grow on the substratum. 5. The spicules are formed at the center of the embryo about two days after fertilization. At first, it is of the monoaxon type, but later becomes tetraxon type by trifurcation o\nf its distal end. Most of the spicules are protriaene, but the spicules in the root belong to anatriaene. 6. Twelve days after fertilization, incurrent cavities are formed under the dermal layer of embryo, through which pores are opened outwardly. 7. The exit of excurrent canal is primarily formed near the base of the root and when the embryo is forty days old after fertilization, the osculum of the adult form is developed on the upper surface of the embryo.