The mesodermic portion becomes charged with a yolk-like material (y), and, on the germination of the statoblast, gives rise to the outer layer (mes) of the bud.
The inner layer of the polypide-bud gives rise to the structures usually regarded as ectodermic and endodermic, the outer layer to the mesodermic organs.
(Adapted from Hatschek.) pc, Praechordal head-cavity of embryo; cc, collar-cavity (first somite); my, mesodermic somites (myocoelomic or archenteric pouches); ch, notochord with the neural tube (neurochord) lying upon it; np, anterior neuropore; ne, position of posterior neurenteric canal.
Y, The yolk-like mesodermic mass.