. The Cambridge natural history. Zoology. vii GEMMULES OF EPHYDA TIA I 79 The mature gemmviles consist of a more or less spherical mass of cells, wliich we shall refer to as yolk cells, and of a complex coat. Tlie latter is provided with a pore or pore tube (Fig. 74) which is closed in winter by an organic membrane. There are three layers in the coat: an inner chitinous layer surrounded by an air - chamber layer, wliich is finely vesicular, showing a structure recalling plant tissue, and con- taining amphidiscs arranged along radii passing through the centre of the gemmule. One of the discs of
. The Cambridge natural history. Zoology. vii GEMMULES OF EPHYDA TIA I 79 The mature gemmviles consist of a more or less spherical mass of cells, wliich we shall refer to as yolk cells, and of a complex coat. Tlie latter is provided with a pore or pore tube (Fig. 74) which is closed in winter by an organic membrane. There are three layers in the coat: an inner chitinous layer surrounded by an air - chamber layer, wliich is finely vesicular, showing a structure recalling plant tissue, and con- taining amphidiscs arranged along radii passing through the centre of the gemmule. One of the discs of each amphidisc lies in the inner chitinous coat, while the other lies in a similar membrane which envelopes the air-chamber layer and is termed the outer chitinous coat. Marshall has suggested that one function of the amphidiscs is to weight the gemmules and thus protect them against the force of the river current; and no doubt ^ the sinking or floating of individual gemmules depends on the relative degree of development of the air- chambers and of the amphidiscs. A study of the development of Ephydatia gemmules vividly illus- trates various characters of the inner fig. 74.—Part of a longitudinal processes of sponges. Specially note- ^^ ^^ IS:^^^^!. worthy are the migrations of cells pore (a). (After Potts.) and the slight extent to which divi- sion of labour is carried : one and the same cell will be found to perform various functions. The beginning of a gemmule is first recognisable ^ as a small cluster of amoeboid archaeocytes in the dermal membrane. These move into the deeper parts of the sponge to form larger groups. They are the essential part of the gemmule, the yolk cells, which, when germination takes place, give rise to a new sponge. They are followed by two distinct troops of actively moving cells. Those forming the first troop arrange themselves round the yolk cells and ultimately assume a columnar form so that they make an epithelioid layer. Tliey then secrete t
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