50 



INVERTEBEATA 



CHAP- 



layer is finally completed no amphidiscs are found, and this forms 

 a weak spot through which the inner mass, which is the real bud, 

 eventually issues forth. The gemmules are set free on the decay of 

 the parent in the autumn, fall to the bottom of the pool or stream 

 and remain dormant till the spring. The inner mass then perforates 

 the weak spot in the membranes, it streams forth in ameoboid 



hvph 



B 



Fig. 27.- 



-Three stages in the formation of the gemmules of Mphydaiia blemiingia. 

 (After Evans.) 



' A, investment of columnar cells incomplete, trophocytes in contact with yolk cells, inner membrane 

 being formed. B, investment of columnar cells complete ; immigration of scleroblast -with amphidiscs. 

 C, ripe gemmule. Yolk cells form a solid mass ; amph, amphidisc ; col, columnar cells ; i.m, inner 

 membrane ; o, aperture for escape of embryo ; o.m, outer membrane ; scl, scleroblast ; tropli, tropho- 

 cyte ; y, yolk cells. 



fashion and forms a little mass of cells which develops- into a 

 sponge. 



Marshall (1884) has shown that in Spongilla lacustris the 

 sponges to which the gemmules give rise reproduce themselves by 

 sexual cells and then perish, whilst the larvae which arise from the 

 fertilized eggs grow into sponges which produce gemmules ; thus there 

 is in this sponge an alternation of generations similar to that with 



