FERTILISATION 27 



extruded, carrying with it a very small portion of cytoplasm. 

 This division is in every sense a division of the one cell 

 the immature ovum into two ; but the two cells are very 

 different in bulk, one of them being many hundred times 

 larger than the other. This small cell is called the ''first 

 polar body " ; the large one is the ovum. The first polar body, 

 when it is outside the ovum, divides into two small cells, 

 which then degenerate. The nucleus remaining in the ovum 

 divides again, and a second small cell is thrown off from the 

 ovum the "second polar body" which also degenerates with- 

 out any further division. The large cell (the ovum) is now 

 ready for fertilisation, and remains passive until this pheno- 

 menon takes place. 1 Sperm cells are generally motile, 

 possessing flagella which enable them to swim in fluids 

 rapidly. As a rule they are several hundred or even thou- 

 sand times smaller than the ovum. The great difference in 

 the mode of production of the sperms and the preparation of 

 the ovum for fertilisation is, that in the case of the former 

 all the cells produced are potential sex cells, while in the 

 latter all the cells resulting from the divisions included 

 in the process of reduction, excepting one, are destined to 

 degeneration and cannot be used in the production of new 

 individuals. 



Fertilisation takes place through a sperm cell reaching 

 the ovum and penetrating into its substance, generally 

 leaving behind it its flagellum. A very important point 

 in fertilisation is, that the centrosomes of the first division 

 appear to be usually derived from the sperm. The centro- 

 somes belonging to the ova degenerate at the time of 

 fertilisation, and so play no part in the cell divisions that 

 subsequently take place. The fertilised ovum thus contains 

 two nuclei, its own and the sperm nucleus. When division 

 takes place in the fertilised ovum the chromosomes appear- 

 ing are derived, half from the ovum and half from the sperm 

 cell. The two daughter cells of the first division of the 



1 The sperm enters the ovum before the extrusion of the polar body in the 

 case of some organisms. 



