In special cases, however, a proglottis may be detached before attaining full growth, and with its generative organs in an imperfectly developed condition.
At this neck-like zone the muscles are absent, and across it falls the line of fracture when the proglottis separates from its fellows.
The reproductive organs are usually repeated in each proglottis, and in some families two complete sets of such organs occur in each segment; in a few cases, parts only of the system are duplicated.
On this view, therefore, at least two asexual generations (embryo and scolex) alternate with a sexual one (proglottides); and in the case of Staphylocystis the cyst contains two asexually produced generations, so that in such forms three stages (embryo, primary scolex-buds, secondary scolices) intervene between the proglottis of a Cestode and that of its offspring.
With regard to the adult worm we have to remember that its two extremities, scolex and terminal proglottis, are different from the intervening region.
The terminal or first-formed proglottis is sterile, and contains the primitive and (except in a few genera) the only excretory pore.