THE REPRODUCTION OF THE PROTOZOA 



107 



division is a very common one in the nuclei of Protozoa, and may 

 show a further advance towards a true mitosis in that the peri- 

 pheral chromatin may shape itself into more or less definite 

 chromosomes, as in Euglena. 



Examples of granular nuclei which divide in the direct method 

 are seen in the division of the nucleus of the oocyst of 

 Coccidium (Fig. 52) to form the nuclei 

 of the sporoblasts (see p. 349, infra) and 

 in the corresponding divisions of the nuclei 

 of haemogregarines (Fig. 53). In these 

 two cases the presence of a centriole in 

 the nucleus is douotful, but is affirmed 

 by Hartmann and Chagas (89) for haemo- 

 gregarines ; a true nuclear membrane, 



FIG. 53. Direct division of the nucleus in the zygote 

 of Hcemogregarina stepanowi. $ , Degenerating 

 male elements attached to the zygote ; N., divid- 

 ing nucleus of the zygote, two successive stages 

 (A and B). After Reichenow (78). 



ev'- 



however, appears to be absent, and this 

 form of division is not much advanced 

 beyond the condition of chromidial frag- 

 mentation. In the macronucleus of 

 Infusoria (Fig. 54), in which a distinct 

 membrane is present, the division is also 

 direct, and centrioles are stated to be 

 absent as a general rule ; in some cases, 

 however, true centrioles appear to be 

 present (Nagler). When centrioles are 

 absent, the achromatinic framework of 

 the nucleus appears to be principally active in the division. In 

 some cases the division of the macronucleus of Infusoria is 

 not into two equal halves, but may take the form of budding 

 off a smaller daughter-nucleus from the main mass. Remark- 

 able instances >i nuclear budding of this kind are seen in the 

 Acinetaria, where it is related to the formation of buds by the parent 

 individual. In some cases (Fig. 55), the nucleus may form a con- 



FIG. 54,, Paramecium cauda- 

 tum : division showing the 

 macronucleus (N) dividing 

 without mitosis, the micro- 

 nucleus (n) dividing mito- 

 tically. C.W. 1 Old, and c.v. 2 , 

 new, contractile vacuoles. 

 After Biitschli and Sohe- 

 wiakoff, in Leuckart and 

 Nitsche's Zoologische Wand- 

 tafdn, No. Ixv. 



