THE STRUCTURE OF CELLS 



33 



very obvious influence on the direction of individual fibrils which 

 may deviate towards and even terminate upon them. 



When the chromosomes have reached the equator of the spindle 

 they may still preserve the form of rings, tetrads, or more complex 

 shapes, and the method of their fission which leads to the severance 

 of the daughter chromosomes is seldom so clearly longitudinal as is 

 the case in a somatic mitosis. The rings present the greatest 

 difficulty, and there exists a considerable divergence of opinion as to 

 whether their division really corresponds to a transverse or to a 

 longitudinal fission of the whole chromosome. The answer practically 

 turns on the conclusions arrived at as to the path of development 

 followed by the chromosome during the earlier phases ; i.e. whether 

 the plane of separation really corresponds to that of the cleavage of 

 the granules, or whether it may not be related to a totally different 

 series of events, and marks the separation of originally bivalent 

 chromosomes as is contended by some observers. In the latter case 

 the complete identity of all the parent somatic chromosomes would 

 be preserved in spite of its apparent loss through the fusion of them 

 in pairs. The separation of the daughter chromosomes would be 

 thus interpreted as not due to the fission of one, but as the segrega- 

 tion of each individual of a pair which had previously become 

 temporarily united. 



After the separation of the daughter chromosomes and the com- 

 pletion of the anaphase (Fig. 15, G) and telophase, the two nuclei 

 which are thus formed commonly commence immediately to divide 

 once more. Again the reduced number of chromosomes reappears, 

 but the character of the process superficially resembles a somatic 

 mitosis much more closely than the preceding heterotype division, 

 and for this reason the name of homotype was given it by Flemming. 

 In reality, however, there are many other points of difference, 

 besides that of the reduced number of chromosomes. The first, 

 and perhaps the chief, peculiarity lies in the fact that there is good 

 reason for believing that the line of fission of the chromosomes is 

 always predetermined during the prophases of the preceding, the 

 heterotype divisions. This is strikingly seen in the case of Ascaris 

 eggs, in which, during the first maturation division, two of the four 

 rods that together represent one chromosome are distributed to 

 the daughter nucleus as the equivalent of a single daughter chromo- 

 some, whilst at the second mitosis each chromosome emerges as a 

 double (not quadruple) body, and at the metaphase the two con- 

 stituent or collective parts separate from each other as the definitive 

 chromosomes of the next (and final) nuclear generation. Essentially 

 the same course of events obtains in cases of tetrads. 



When these divide at the equatorial plate the resulting dyads 

 thus formed retreat as the daughter chromosomes, and on the rapidly 

 following homptype division the dyads again reappear in the early 



