262 



THE PROTOZOA 



a 



he described two distinct parts, one homogeneous, and with the 

 exception of a central granule, which he called the " nucleolus," with- 

 out structure ; the other, chromatin in the form of granules, which 

 surround the homogeneous portion. In preparation for budding, the 

 granule within the homogeneous portion swells, sends out pseudopodial 

 processes, and at the same time becomes more indistinct, until, finally, 

 the substance of the so-called nucleolus is lost in the surrounding 



granules of chromatin. Shortly 

 afterward, two heaps of homo- 

 geneous substance (pole-plates} 

 appear at the ends or poles of 

 the division-figure, and the 

 chromatin granules are arranged 

 in lines between these masses. 

 Even at this early date Hertwig 

 compared the pole-plates or 

 "end-plates," as he called them, 

 with the Polkorperchen (centro- 

 somes) in the mitotic figures of 

 Metazoa (Fig. 139, A-C), and 

 concluded that they are derived 

 from the original " nucleolus." 



Spirochona has been re- 

 peatedly examined since 1877, 

 and Hertwig's main conclusions 

 are confirmed. There is a dif- 

 ference of opinion, however, in 

 regard to the origin of the 

 "nucleolus" or division-centre, 

 as we may be justified in calling 

 it. Plate ('86) thought that the 

 corpuscle is formed anew after 

 division, by the accumulation of 

 chromatin which penetrates the 

 homogeneous portion in a state of solution. He also distinguished 

 an inner structure in the supposed homogeneous portion, and main- 

 tained that the pole-plates are formed from its substance. Balbiani 

 ('95) more recently came to a somewhat similar conclusion. He 

 found that the so-called homogeneous part is made up of short and fine 

 fibrillae which do not stain with the chromatin dyes, and are, there- 

 fore, to be classed as "achromatic" structures. He maintained that 

 the " nucleolus " is formed by the aggregation of several granules 

 of chromatin during the later stages of division. 



Two substances, therefore, are present in the nucleus of Spirochona, 



Fig. 138. a. Macronucleus and micronucleus 

 of Stylonychia in division, b. Micronucleus of 

 Paramoecium aurelia in division, c. The nucleus 

 of Chilodon cucullulus. [BUTSCHLI.] d. Dividing 

 nucleus of Clepsidrina sp. 



