GROWTH AND REPRODUCTION 153 



stationary and relatively large, the female ; the other smaller and 

 motile, the male. In the unicellular organisms, the whole organism 

 is frequently converted into one female gamete, or into a number of 

 male gametes. The latter swim freely, and one of them conjugates 

 with a female gamete. But in the multicellular animals and plants, 

 special organs are formed for the purpose of producing the two kinds 

 of gametes. The female gametes are now called ova or egg-cells ; 

 while the male gametes have different names, spermatozoa in 

 animals, antherozooids or pollen grains in plants (E., p. 226). The 

 process corresponding to the conjugation of the simple organisms 

 is now called "fertilisation" The organism itself as a whole 

 becomes modified, the ova-bearing or female organisms being 

 different in many ways from the male; in some cases extraordinarily 

 so. The organ in which the ova are formed is known as the "ovary" \ 

 that in which spermatozoa are formed is the testis. 



The material from which the sexual cells are formed in the 

 course of the development of the young organism from the fertilised 

 ovum is very early separated from that which becomes differentiated 

 into the various organs of the body. It thus retains the whole of 

 the characters of the gametes from which it has resulted, and, even 

 in the adult, is but little affected by changes in the rest of the 

 organisms. In a certain way we may speak of the "continuity of 

 the germ plasm " from generation to generation. The question as 

 to how far it can be influenced by changes affecting other parts of 

 the organism will be discussed later. 



When two similar cells conjugate there is, along with the general 

 admixture of cell substance, a dissolution of the nuclei, followed by 

 formation again of a single mixed nucleus. In the process of 

 formation of the male and female gametes in the true sexual process, 

 on the other hand, half the nuclear material is thrown off, so that 

 the final fertilised ovum contains the normal amount, half of its own, 

 the other half derived from the male element. 



Although the process of fertilisation described above is the 

 normal one, the ovum until fertilised remaining stationary with no 

 cell division or growth of the new organism occurring, it is remark- 

 able that, in some exceptional cases, the unfertilised ovum is able 

 to develop. In such a case as that of the bee, this so-called 

 " parthenogenesis " is a normal fact, the fertilised cells becoming 

 the worker bees, the unfertilised cells becoming the males, called 

 "drones." In other cases, such as the sea-urchin, the ova, although 

 in the normal state of affairs requiring fertilisation, can be 

 stimulated to development by chemical or physical means of 

 various kinds. 



In order that the ovum may become fertilised, it is clear that 

 the spermatozoa must obtain access in some way. When the eggs 



