i88 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



to take tin- form of stout curved rods (Fig. 85, D). The bound- 

 - of tlu- two nuclei disappear so that the chromosomes lie free in 

 the cvtoplasm, and a spindle makes its appearance, fibres passing from 

 the chromosomes which lie about its equator to each centrosome and 

 radiating out from the centrosome into the surrounding cytoplasm. 

 h chromosome splits longitudinally (Fig. 85, E) and its two halves 

 slowly recede towards opposite poles of the spindle (Fig. 85, F). Thus 

 there travel towards each pole four daughter chromosomes,, two of which 

 are <>l paternal origin derived from the sperm chromosomes, two 

 oi maternal derived from the egg chromosomes. The egg now 

 imes surrounded by a furrow round its equator which gradually 

 >i us until the egg is completely divided into two daughter cells. 

 This division is the first step in the development of the new individual, 

 the first step in the process known as the segmentation of the egg. The 

 important point to notice is that the chromosomes in each of the two 

 dau-hter cells or blastomeres are diploid in number, and are half of 

 puti-mul and half of maternal origin. Throughout subsequent develop- 

 ment, as the blastomeres divide over and over again to form the immense 

 - of cells constituting the adult body, the process of splitting is 

 repeated at every mitosis, so that each cell in the body contains 

 nuclear material derived equally from the two parents. 



When eventually, in the gonad of the new individual, the process of 

 -iyndesis takes place there is reason to believe that the two chromosomes 

 that come together are one of paternal origin and one of maternal. The 

 evidence on which this belief is based comes not iromAscaris but from 

 other animals but it is possible to indicate in a few words its nature. 



It has been possible by detailed study of the chromosomes of various 



animals to determine that any particular species is characterized not 



merely by the definite number of its chromosomes but also by definite 



rs of the individual chromosomes. Thus in the developing 



the haploid group is constant not merely in its number but in 



imposition. It is made. up of a definite assemblage of chromosomes 



mall differences in size and shape which make it possible to 



them individually and to label them with definite designations, 



ttCTS of the alphabet. Thus supposing the haploid number is six 



possible to distinguish a. b, c, d, e, f, each characterized by 



lr finite shape and size. In each haploid group we find the same set of 



recurring each recognizable by its special peculiarities. It 



iv there is present in the diploid group a double 



2!), 2c, 2d, 26, 2f. The corresponding chromo- 



' hromosomes for example are known technically 



