8 THE INITIATION OF DEVELOPMENT. 



exit groups of animals, but usually we can distinguish a head, 

 representing the nucleus of the cell, a middle piece, and a long, 

 motile tail. The undulating movements of the latter propel 

 the sperm. 



In all organisms, the nucleus of each cell contains a number 

 of small bodies with a high affinity for stains, the chromo- 

 somes. They become visible each time the nucleus divides. 

 Within each species, their number is constant. We shall see later 

 that they are very important for the life of the organism 

 (Ch. VI). During fertilisation the nuclei of egg and sperm fuse 

 (amphimixis), so that a double number of chromosomes is 

 present in the nucleus of the fertilised egg, and likewise in that 

 of all the cells of the new individual, which arise from it by 

 cell division. This would lead to doubling of the chromosome 

 number in each successive sexually reproduced generation, but for 

 the fact that during the formation of the germ cells the number 

 is each time reduced again to the single value. This takes place 

 in the so-called reduction division, actually two divisions in 

 quick succession in the course of which the double (diploid) 

 set of chromosomes is reduced to a single (haploid) set. In the 

 formation of sperms, these reduction divisions take place in 

 the male gonad, or testis, one diploid spermatocyte giving rise 

 to four haploid spermatids which then change into spermatozoa. 

 In eggs, the reduction process takes a slightly different course. 

 Here it is known as maturation, and proceeds as follows. The 

 nucleus of the oocyte moves towards the surface; its nuclear 

 membrane disappears, and a mitotic spindle develops. Mean- 

 while each chromosome has duplicated itself, and the double 

 chromosomes now arrange themselves in pairs in the centre 

 of the spindle (Fig. la). The two members of each pair then 

 move apart, so that two equal groups of double chromosomes 

 are formed. One of these groups, together with a small quantity 

 of protoplasm, is expelled from the egg, and forms the first 

 polar body (Fig. lb). Immediately afterwards, a new spindle 

 forms in the egg, and the remaining double chromosomes 

 (dyads) arrange themselves in the centre of this spindle. The 

 two halves of each dyad separate (Fig. Ic). One group is ex- 

 pelled again, forming the second polar body. From the remain- 



