i88 ZOOLOGY FOR MEDICAL STUDENTS chap. 



up so as to take the form of stout curved rods (Fig. 85, D). The bound- 

 aries of the two nuclei disappear so that the chromosomes He free in 

 the cytoplasm, 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. 

 Each 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 of paternal origin — derived from the sperm chromosomes, two 

 (iV) of maternal — derived from the egg chromosomes. The egg now 

 becomes surrounded by a furrow round its equator which gradually 

 deepens 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 

 daughter cells or blastomeres are diploid in number, and are half of 

 paternal and half of maternal origin. Throughout subsequent develop- 

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

 mass 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 

 syndesis 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 from Ascaris 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 

 characters of the individual chromosomes. Thus in the developing 

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

 its composition. It is made up of a definite assemblage of chromosomes 

 showing small differences in size and shape which make it possible to 

 recognize them individually and to label them with definite designations, 

 such as letters of the alphabet. Thus supposing the haploid number is six 

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

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

 chromosomes recurring — each recognizable by its special peculiarities. It 

 follows that after syngamy there is present in the diploid group a double 

 set of chromosomes — 2a, 2b, 2c, 2d, 26, 2f. The corresponding chromo- 

 somes — the two " a " chromosomes for example — are known technically 



