Have become obliterated by coalescence of their walls, so that the entire endoderm of the umbrella is in the condition of the endodermlamella.
In some hydroids the founder-polyp, developed from a planula after fixation, throws out numerous outgrowths from the base to form the hydrorhiza; these outgrowths may be radially arranged so as to form by contact or coalescence a flat plate.
The development, on the contrary, shows unequivocally that the endodermlamella arises as a local coalescence of the endodermal linings of a primitively extensive gastral space.
By coalescence of the endoderm-layers, the coelenteron may be reduced to vessels, usually eight in number, opening into a ring-sinus surrounding the pore.
Laticiferous vessels arise by the coalescence of originally distinct cells.
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