32 THE CHROMOSOMES 



chromatid are forced to go to the same pole at ana- 

 phase. ^^^ This Ijappens in the spermatogonia! and 

 oogonial divisions and at the first cleavage division. 

 In the later cleavage divisions, however, a different 

 type of division takes place in all those blastomeres 

 in which are going to give rise to the somatic tissues 

 of the adult — here the central region of the chromo- 

 somes breaks up into a number of much smaller 

 chromosomes, each of which probably has a single 

 spindle attachment. The ends of the chromosomes 

 are left with no spindle attachments and do not 

 become connected with the spindle in any way but 

 degenerate in the cytoplasm. This process results in 

 an organism whose germ-cells contain two or four 

 (according to the variety) large chromosomes, while 

 the somatic cells have a much larger number of small 

 chromosomes (the exact number of which is still in 

 doubt). 



In another species of Ascaris, A. lumhricoides , the 

 ends of the chromosomes are broken off and degen- 

 erate in the <3ytoplasm in the same way, but there is 

 no fragmentation of the central region. ^^ 



Two other types of chromosomes may be men- 

 tioned here which have been observed on a number 

 of occasions, but do not appear to have become per- 

 manent in any wild species of organism. The first 

 of these is the branched chromosome. It was formerly 

 believed on genetical evidence that the 'pale ' mutation 

 of Drosophila melanogaster was due to a small piece 

 of the second chromosome having been broken off 

 and attached to the side of the third one. This 

 explanation has^now been abandoned as far as the 

 pale mutation is concerned, but undoubted cases of 

 branched chromosomes have been seen in several 

 organisms. 2' » ^^ In most of these cases, the side- 

 branch was joined on to the main chromosome at 

 the spindle attachment. 



The other type of chromosome is the ring chromo- 

 some , which has been found on a number of occasions, 



