1917] Boeck: Mitosis in Giardia microti 13 



motomy. A large central, ellipsoidal karyosome, connected by an 

 achromatic fibril, linin in nature, to the centrosome (pi. 1, fig. 1 ; text- 

 fig. A) is characteristic of each nucleus in the resting stage. The first 

 mitotic change in the prophase consists of the extension of this linin 

 fibril posteriorly to a point on the nuclear membrane (pi. 1, fig. 1; 

 text-fig. B) and the expansion of the chromatin of the karyosome to 

 form a long, slender, ellipsodial karyosome. 



It is to be noted that thus far the nuclear changes have been of 

 such a nature as to make more pronounced a polarity in the nucleus. 

 This is marked by the extension of the linin fibril posteriorly and the 

 expansion of the chromatin anteriorly and posteriorly on it. This 

 polarity of the nucleus can be followed in later stages. It is related 

 to the polarity exhibited by the cytoplasmic structures, especially to 

 the main axial structure, the axostyle, which defines the poles and 

 main axis of the body of the organism itself, although the axes of the 

 two nuclei are not quite parallel to this major axis of the body. The 

 longitudinal and what appears to be an equivalent splitting of this 

 spireme to form two single, somewhat ragged spireme strands in each 

 nucleus soon follows (pi. 1, fig. 3; text-fig. D). The splitting begins 

 in the area adjacent to the centrosome and proceeds posteriorly ajid 

 completely through the spireme band. These two spireme strands in 

 each nucleus soon show the beginnings of the differentiation of the 

 chromosomes by localized knotting up of the substance, which exhibits 

 itself first at the end nearest the centrosome (pi. 1, fig. 3, right 

 nucleus; text-fig. D). Thus from this progressive action in the split- 

 ting of the spireme and in the formation of the chromosomes, the cen- 

 trosome, the center of mitotic activity, exerts its influence on the 

 mitotic process. 



After the formation of the split spireme, localized contraction and 

 tranverse segmentation of the chromatin on these strands ensues to 

 form four chromosomes from each strand, or eight chromosomes for 

 the tetraploid number of each nucleus (pi. 1, fig. 6; text-fig. F). An 

 intermediate stage shows the chromosomes first completely formed 

 nearest the centrosome, again giving evidence of the fact that mitotic 

 activity proceeds from the still undivided centrosome, as though 

 under its influence, towards the opposite pole. The chromosomes are 

 seen to be connected end to end by linin fibrils, forming two chains of 

 four chromosomes each. These chains are in most cases parallel to 

 the major axis of the nucleus, thus maintaining the same polarity 

 (pi. 1, figs. 4, 5, 6). 



