The third or outermost chamber, the proctodaeum, is closed externally by the sphincter ani; the orifice is quite circular.
In such cases the blastopore may entirely close, and both mouth and anus develop as new ingrowths (stomodaeum and proctodaeum), whilst, according to the observations of N.
The anal opening forms at a late period by a very short ingrowth or proctodaeum coinciding with the blind termination of the rectal peduncle (fig.
Further, there is general agreement that the chitin-lined fore-gut and hind-gut, which form the greater part of the digestive tract, arise from ectodermal invaginaof the germ band a double fold in the undifferentiated blastoderm, tions (stomodaeum and proctodaeum respectively) at the positions which grows over the surface of the embryo, so that its inner and of the future mouth and anus.
In some parasitic Hymenoptera there is only a single the proctodaeum respectively.
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