THE SUN ANIMALCULE 153 



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 secondary cysts 

 have become gametes and forthwith proceed to conjugate. 

 The remarkable feature in Actinosphaerium is that conjugation 

 is limited to the original pairs formed by the division of the 

 primary cysts. The members of each pair fuse together again, 

 nucleus with nucleus and cytoplasm with cytoplasm, to form 

 zygotes, the number of which is, of course, equal to that of 

 the original primary cysts. Each zygote becomes a spore 

 with a firm spore-coat of two layers ; externally a siliceous 

 envelope formed by the union of the loosely scattered spicules 

 of previous stages, and within this a tough resistant mem- 

 branous 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 immediately 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 Actinosphaeria, which proceed forthwith to feed and 

 grow, the single nucleus dividing to form many as growth 

 proceeds. 



