The bud arises as a disc-like thickening of the anterior atrial wall, consisting of a small number of columnar cells transformed from the atrial epithelium, overlain by an equivalent area of unmodified epidermis. The polarity of the disc and subsequent organism is an extension of that of the parental tissue, with regard both to the antero-posterior and lateral axes. Development itself is fundamentally extremely simple and direct. After the completion of development there is a phase of functional activity and a phase of autolysis and dissolution. For any given time-temperature scale the duration of these last two phases is as specific and determined as that of the developmental phase. Of the two tissues constituting the bud disc the epidermis forms only more epidermis, though acquiring the form of a whole organism including the ventral stolonic outgrowth. The atrial component of the disc forms everything else. As the disc expands, by means of cell multiplication, it transforms progressively into a hemisphe...