Fourth division is horizontal and unequal resulting into 16 blastomeres that are arranged in two tiers. 8 cells of lower tier are larger and are called macromeres. They form pinnacoderm in the adult. Other 8 cells of upper tier are smaller and are called micromeres.
They form the choanoderm. Micromeres undergo subsequent division rapidly to produce numerous micromeres. They all acquire flagella at their inner ends, facing the blastocoel. Macromeres don’t undergo division but become somewhat rounded.
The embryo is now called blastula; Duboscq and Tuzet have preferred to call this stage as stomoblastula. (b) Stomoblastula: In this stage 8 rounded and nonflagelated macromers are flanked over by numerous small, elongated and flagellated micromeres. The flagella of micromeres face into blastocoel which opens outside through mouth.
Mouth is situated in the centre of macromeres and is used for engulfing the surrounding amoebocytes for nutrition. (c) Amphiblastula: Amphiblastula is a free swimming larva of Sycon that comes out from the mesenchyme into the spongocoel. It finally passes out from the spongocoel through osculum. It develops from stomoblastula by inversion, in which it turns itself out through the mouth, so that the flagella of micromeres become directed outwards. Now the embryo acquires a look in which its one half bears flagella and the other half does not. The embryo is now called amphiblastula (Gr., amphi, both + blastos, germ,) having two kinds of blastomeres. Amphiblastula larva swims freely in water for some time.
While swimming, the flagellated pole is directed anteriorly and the force for swimming is supplied by the beating of flagella. (a) Gastrula: After some time, amphiblastula attaches to some substratum and undergoes gastrulation. In this process the macromeres multiply more rapidly than micromeres, and grow over the flagellated half. The larva now resembles a typical double walled gastrula; Outer wall consists of non-flagellated micromeres and inner wall consists of flagellated micromeres. It opens through mouth, called blastopore.
(b) Post larval period or metamorphosis: At the blastoporal end gastrula fixes itself and lengthens into a cylinder. At the free distal end osculum is formed and numerous small ostia form in the wall of cylinder. Outer nonflagellated granular cell wall forms pinnacoderm, scleroblasts and porocytes in the future adult stage. While inner flagellated cell develops into choanoderm and gives rise to functional choanocytes, archaeocytes and other amoebocytes. Mesenchyme cells are thus derived from both the embryonic layers. This young cylindrical form of scypha is called olynthus larva and resembles a simple ascon sponge.
From the middle part of choanoderm of olynthus radial canals start to develop by budding. Folding occurs, within the body walls as the growth of larva continues. Sycon type canal system finally develops in the young sycon and it forms colony by further branching and differentiation in the course of time.