vii GEMMULES OF EPHYDATIA 1 79 



The mature gemmules consist of a more or less spherical 

 mass of cells, which we shall refer to as yolk cells, and of a 

 complex coat. The latter is provided with a pore or pore tuhe 

 (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, which 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- sect ? on of a s emmule f phy- 



x 1 p datia sp. passing through the 



worthy are the migrations of cells pore (). (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 l 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 anol ultimately assume a columnar form so 

 that they make an epithelioid layer. They then secrete the inner 

 chitinous coat. The cells of the second troop are entrusted with 

 the nutrition of the gemmule. Consequently they pass in among 

 the yolk cells, distribute their food supplies, and make their escape 



1 Evans, Quart. Journ. Micr. Sci. xliv. 1900, p. 72. 



