378 



THE INFUSORIA 



spindle is, in this condition, frequently very much elongated, 

 stretching more than half-way across the medulla. In Dendro- 

 cometes it is at this stage as much as 30 /A in length (Fig. 31). The 

 spindle disappears suddenly and seems to be completely absorbed 

 by the cytoplasm immediately, or very soon after, it is disconnected 

 from the chromatin granules at the poles. 



M 



* - grr--- rn 



THE NUCLEI OF OPALINA. 



If the current views concerning the nuclei of Opalina are trustworthy, 

 this genus should no longer be regarded as a member of the Hetero- 

 karyota. Opalina possesses, according to Pfitzner and others, a large 

 number of meganuclei, but no micronuclei. Moreover, the meganuclei 

 divide by a typical process of mitosis. These views may possibly be 

 erroneous. Thin sections of Opalina that are suitably stained show, in 

 addition to the numerous meganuclei, a large number of small bodies 

 containing chromatin. These are probably 

 micronuclei (Fig. 25). The meganuclei divide 

 sometimes amitotically, and it is probable that 

 ^y always do so. The mitotic figures dis- 

 covered by Pfitzner are clearly seen in a large 

 number of sections examined, but they are 

 smaller than the meganuclei and are probably 

 formed by micronuclei which, as in other forms, 

 increase considerably in size before division. 

 The matter requires, however, further investi- 

 gation. 



CONTRACTILE VACUOLES. The contrac- 

 tile vacuoles occur in all Heterokaryota 

 except in Opalina, Opalinopsis, some of the 

 marine Hypotricha, and a few others. They 

 are simply spaces formed at some localised 

 spot or spots in the medulla by the accumu- 



lation of a fluid > and the y dischar g e their con - 



?i?? ?ll large me s an . uclei tents to the exterior when they have reached 



(Af) and the numerous minute ... J . 



chromatin bodies (m\ which a definite limit of size. I he fluid that ac- 

 e micronuclei ' cumulates in these spaces is probably charged 

 with waste products of the metabolism 

 of protoplasm. Although the positions in which these vacuoles 

 appear are fairly constant for each individual, they have no 

 proper walls, and must not be regarded as definitely formed 

 organs. The number varies enormously in the group (1-100 or 

 more), and when there are more than three or four in any one 

 species the number may vary in the individual (Fig. 26). In some 

 of the large species the number of the contractile vacuoles increases 

 with the size and age of the individual, but in Nassula, Dendro- 



FlO. 25. 



