14 University of Calif ornia Publications in Zoology [Vol. 18 



It is significant to note here that the chromosomes arrange them- 

 selves into two groups of four each and that the line of separation is 

 at the equator of the nucleus (pi. 1, fig. 5; text-fig. F). This suggests 

 the possibility of a biparental origin of the chromosomes, and indicates 

 the probability of the occurrence of sexual reproduction in Giardia, 

 but there is as yet no evidence for this assumption. The linin con- 

 necting fibrils between chromosomes are present at this stage of 

 mitosis. 



The chromosomes appear to be oblong in shape, about 0.3/* long 

 and about 0.2/a in width. The chromosomes derived from a single 

 spireme strand in a nucleus show immediately after their formation a 

 tendency to pair off with the adjacent chromosomes derived from the 

 other spireme strand of the same nucleus (pi. 1, fig. 5; text-fig. F). 

 The chromosomes appear to be very much alike in shape and size. 

 More evidence for polarity within the nucleus is displayed by the 

 chromosomes in that their long axes are parallel to the long axis of 

 the nucleus itself, as the chromosomes are differentiated in two linear 

 axial lines in the nucleus (pi. 1, fig. 5; text-figs. F, G). 



Thus far it must be noted that all these mitotic changes, including 

 the formation of the eight or tetraploid number of completely divided 

 chromosomes, have taken place before the division of the centrosome 

 or any evidence of activity therein, and that throughout the process 

 thus far all nuclear changes have consistently displayed a polarity 

 related to that of the polarity of the nucleus itself and of the organism 

 as a whole in the direction comparable to that of the axial gradient 

 of Child. 



On the division of the centrosome of each nucleus at this time, one 

 daughter centrosome remains fixed while the other one migrates to a 

 point on the periphery of the nucleus at the opposite pole 180° from 

 its original situation, and here marks the posterior pole of the spindle 

 to be formed later (pi. 1, fig. 5; text-fig. G). A paradesmose lying 

 outside the nucleus but closely applied to the nuclear membrane is 

 formed as a connecting fibril between the two daughter centrosomes. 

 , The chromosomes at this stage, as has been previously stated, may 

 often be seen to be still connected in an end-to-end manner by linin 

 fibrils, but there is a tendency now for them to disperse throughout 

 the nucleus ; this progressive stage and the one preceding are often 

 found in the two nuclei of the same individual (pi. 1, fig. 5). Because 

 of this dispersal there is given the opportunity for a rearrangement 

 of the chromosomes different from that of their first order, but there 



