VII GE^^mULES OF EPffVD A T/.4 l^g 



The mature geinmules consist of a more or less splierical 

 mass of cells, which we sliall 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 cliitinous 

 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 tlie 

 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. 



Marsliull has suggested that one function of the amphidiscs is 

 to weight the gemmules and thus protect tliem 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 tlie amphidiscs. 



A study of the development of 

 EphydcUict gemmules vividly illus- 

 trates various characters of the inner fig. 74.— Part of a longitudinal 

 processes of sponges. Specially note- «f ^.i°'^ «*' "" gemmi^ie of E^hy- 



'■ , . . f. n 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 gennnule 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. 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 



' Evans, Quart. Journ. Micr. Scl. xliv. 1900, p. 72. 



