THE REPRODUCTION OF THE PROTOZOA 



115 



the mechanism of division, is furnished by some examples of the 

 Heliozoa, and especially by the nuclear divisions of Actinosphcerium, 

 which have been the subject of extraordinarily thorough investiga- 

 tion by Hertwig (64). In this form there are three different modes 

 of karyokinesis, which, however, for present purposes may be 

 classified under two heads : karyokinesis without and with centro- 

 somes. In the ordinary nuclear division during the vegetative life 

 of the organism, and also in the divisions by which the primary 



G 



FIG. 62. Actinosphcerium eichhorni : stages of the ordinary .'vegetative nuclear 

 division, without centrosomes.^of free-living individuals (not encysted). 

 A, B, Formation of the chromosomes within the nucleus, and of the "proto- 

 plasmic polar cones outside the nucleus ; G, spindle-stage with polar cones 

 K.C.), polar plates (p.p.), and equatorial plate of chromosomes (e.r.); 

 stage with daughter - plates of chromosomes which have travelled 

 towards the polar plates ; E G, division of the nucleus, reconstitution of 

 the daughter-nuclei, and disappearance of the polar cones. After Hertwig (64). 



cysts divide into the secondary cysts (p. 138), centrosomes are 

 absent, but they are present in the two divisions which produce 

 the two reduction - nuclei thrown off from each secondary 

 cyst. 



In the ordinary karyokinesis of Actinosphcerium (Fig. 62) an 

 equatorial plate is formed composed of a large number of small, 

 rod-like chromosomes, imperfectly separated from one another, 

 which divide transversely. The spindle arises from the achromatinic 

 framework of the nucleus, and terminates in two conspicuous polar 



