18 COMPARATIVE ANATOMY chap. 



VII. Stigmata (red eye-spots). 



Stigmata are found, generally singly, in many coloured Flagellata. 

 It is highly doubtful if these are organs sensitive to light. 



VIII. Nuclei. 



These seem to be absent in the Monera. In all the other Protozoa 

 they are found, either singly (many Amceha, some Heliozoa, all Gregarina, 

 and most Flagellata and Infusoria) or in numbers. They lie in the 

 endoplasm — in the Radiolaria in the central capsule, in the Gregarina 

 in the deutomerit — and are either vesicular, with membrane, sap, and 

 one or more nucleoli, occasionally with a sort of nuclear framework, 

 or homogeneous. They vary greatly in shape. The nuclear processes 

 in most Infusoria present complications. We here find a double 

 nucleus, viz. a large maeronueleus, and, lying more or less close to 

 this, a small micronueleus (formerly erroneously called nucleolus). 

 The micronueleus plays an important part in conjugation ; the 

 maeronueleus, on the contrary, during this process falls to pieces and 

 degenerates in a peculiar manner. When conjugation begins, the 

 micronueleus divides twice, i.e. into four parts. Three of these four 

 parts disappear, while the fourth again divides into two nuclei — the 

 migratory nucleus and the stationary nucleus. The most important 

 process during conjugation is the mutual exchange of the migratory 

 nuclei of the two conjugating individuals A and B. The migratory 

 nucleus of A passes over into B and fuses with the stationary nucleus 

 of B, while the migratory nucleus of B passes over into A to fuse with 

 the stationary nucleus of A. A new maeronueleus and a new micro- 

 nucleus arise, in the individuals which separate after conjugation, out 

 of the nucleus which results from the fusing of the migratory and 

 stationary nuclei. 



The division of the nucleus is either a direct division (constriction, 

 dumb-bell stage, separation into halves), or it resembles the indirect 

 nuclear division found among Metazoa, which will be described later. 



IX. Reproduction. 



The phenomena of reproduction among the Protozoa deserve more 

 detailed investigation, as we find included under this head a tolerably 

 complete series of intermediate stages between the simplest reproduc- 

 tion by fission and sexual reproduction. 



Reproduction by simple binary fission (cross, longitudinal, and 

 diagonal fission) takes place in nearly all divisions of the Protozoa. 

 It is especially characteristic of the Monera, many Ehizopoda, many 

 Flagellata, and all Infusmia. It is, however, not observed in the 

 Gregarina. 



Reproduction by budding or gemmation is, in the simplest cases. 



