146 COMPARATIVE ANATOMY 



halves is expelled from the cyst as the first polar body. The 

 other half increases in size and again divides mitotically to 

 form the second polar body. Both polar bodies consist only 

 of nuclear material, and speedily disintegrate and disappear. 

 After the extrusion of the polar bodies the members of each 

 pair of secondary cysts fuse together again, nucleus with 

 nucleus and cytoplasm with cytoplasm, to form as many 

 spores as there were primary cysts. The spores, however, 

 differ from the primary cysts in being denser and more opaque, 

 in having firm spore coats, formed by the union of the loosely 

 scattered siliceous spicules of the previous stages, and in having 

 a tough resistant membrane within the siliceous coat. The 

 spores remain in this condition for weeks together, and then on 

 the onset of warmer weather the spore coats, and also the 

 gelatinous cyst wall, are ruptured, and from each spore a young 

 Actinosphaerium emerges. These young forms are vacuolated 

 and protrude pseudopodia. Each has several nuclei, formed 

 by mitotic division of the single spore nucleus either imme- 

 diately before or during emergence, but it would appear that 

 before growing any further it divides into as many pieces as 

 there are nuclei, so that the end result of this long and 

 complicated process is a brood of young uni-nuclear Actino- 

 sphaeria, which proceed forthwith to feed and grow, the single 

 nucleus dividing to form many as growth proceeds. 



