The life history of Diplodiscus temporatus Stafford. 651 



tral sucker; otherwise only an open tube of the cyst forming- material 

 will result. 



3. The cercariae are positively heliotropic, and when set free 

 from the snail in a normal manner, always swim to, or near to, the 

 surface before encystment takes place. 



4. The encysted worms are unable to withstand dessication for 

 more than one hour. 



5. Tadpoles, or other animals which obtain their food by brow- 

 sing along" the banks of streams, may take up the encj^sted cer- 

 cariae with their food. In large tadpoles the worms become 'trans- 

 formed into sexually mature worms. In other animals, and in some 

 cases at least in small tadpoles, the worms become again encysted. 

 If such intermediate hosts are eaten by adult amphibians the worms 

 would be able to go on with their development again. 



6. The eggs (germ cells j in the sporocyst may arise either from 

 cells which have come over from an early developmental stage free 

 in the body cavity of the sporocyst, or they may as is the case 

 throughout the later life of the sporocyst, arise from nuclei in the 

 body wall which become segregated at one end of the body cavitj' 

 to form a rudimentary ovary. 



7. The mitosis in this form is intranuclear. All of the chro- 

 matin in the nucleus is gathered into a single (rarely there are two) 

 caryosome. This body is surrounded by a densely staining mass of 

 cytoplasm. The centrosomes in the resting stages of the nucleus 

 lie either in the body of the caryosome, or so closely applied to it 

 that their identity cannot be made out. The centrosomes become 

 apparent in a comparatively late prophase. 



In the prophase, the caryosome increases in size at the ex- 

 pense of the nucleoplasm until the whole of the nuclear contents 

 are gathered into a single, lightly staining, mass which is con- 

 nected with the nuclear membrane by several strands. Within 

 this,, at first homogenous mass, there appear chromatin granules 

 which later increase in size and finally fuse to form the spireme 

 thread, which soon segments to form the chromosomes. 



The spindle fibres are very prominent, each one being apparently 

 made up of several (four) smaller fibres. 



up to the metaphase of division the nuclear membrane retains 

 its nearly spherical shape. The elongation begins in the anaphase 

 and the constriction follows immediately. 



In the anaphase the daughter chromosomes are drawn up into 



