580 SUMMARY OF CURRENT RESEARCHES RELATING TO 



ford, with especial reference to the development of the parfchenogenetic 

 eggs. The redige and cercariae in this form show many marked differ- 

 ences from the same stages in the European species, Amphistomum 

 subclavatvm. with which it has usually been confounded. For the 

 formation of a complete cyst it is essential that the cercaria be able to 

 attach itself to some substratum by means of the ventral sucker, other- 

 wise only an open tube of the cyst-forming material will result. The 

 cercaria? are positively heliotropic, and, when set free from the snail in 

 the normal manner, always swim to or near to the surface before encyst- 

 ment takes place. The encysted worms are unable to withstand 

 desiccation for more than one hour. Tadpoles or other animals, which 

 obtain their food by browsing along the banks of streams, may take up 

 the encysted cercariae with their food. In large tadpoles the worms 

 become transformed into sexually mature worms. In other animals, 

 and in some cases in small tadpoles, the worms become again encysted. 

 If such intermediate hosts are eaten by adult Amphibians, the worms 

 are able to go on developing again. The eggs (germ-cells) 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-cavity to form a rudimentary ovary. 



The mitosis in this form is intranuclear. All the chromatin in the 

 nucleus is gathered into a single caryosome. (Rarely there are two.) 

 This body is surrounded by a densely staining mass of cytoplasm. The 

 centrosomes in the resting stage of the nucleus lie either in the body of 

 the caryosome, or so closely apposed 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 

 expense of the nucleoplasm until the whole of the nuclear contents are 

 gathered into a single lightly staining mass, which is connected with 

 the nuclear membrane by several strands. Within this at first homo- 

 geneous 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 con- 

 striction follows immediately. In the anaphase the daughter-chromo- 

 somes are drawn up into a compact mass in which all outlines of the 

 separate chromosomes are soon lost. The constriction of the nuclear 

 membrane continues until the daughter-nuclei are separated as if by 

 amitosis. The formation of the spireme for the maturation division 

 takes place without the concentration of the whole of the nuclear con- 

 tents into an enlarged caryosome. Instead, the caryosome becomes 

 directly converted into the spireme thread without the intervention of a 

 stage in which the chromatin exists in small granules. Usually the 

 maturation spindle is excentric in the egg ; it is located near that end 

 of the nucleus which lies nearest to the cell-wall. In the formation of 

 the polar body a portion of the nuclear membrane containing one set 



