INTRODUCTORY 3 



layer lining the gut becomes concerned with the digestion and 

 absorption of the food ; while between these the skeleton and 

 general framework of the body are developed, besides all the 

 other organs and tissues which contribute to the performance of 

 the vital functions. Amongst these are the generative organs or 

 gonads which differ according to sex, male animals having testes 

 and female ones ovaries, in which the actual reproductive cells, 

 or spermatozoa and ova, are respectively produced. Thus in the 

 bodies of the higher animals— and the same is true of the higher 

 plants — certain specialised cells are produced, the function of 

 which is to unite and to reproduce. In the process of fusion 

 as it occurs in all multicellular animals a single spermatozoon 

 unites with a single ovum, the two nuclei also uniting, and this 

 fusion of the generative cells or gametes to form one cell, known 

 as the zygote, is the essential act in sexual reproduction. 



Asexual Reproduction. — As we have seen already in unicellular 

 organisms like the amoeba reproduction takes place by binary 

 fission, without any further complicating process. This method 

 is the simplest form of asexual reproduction. The two products 

 of division go on growing until they have reached a certain size, 

 and then they divide again, and so the process is repeated. In 

 some multicellular animals also reproduction takes place by binary 

 fission. Thus in the sea anemone the whole body occasionally 

 divides into two equal portions, and the same is true for certain 

 small flat worms when nutritive conditions are especially favour- 

 able. In such cases the products of division go on growing until 

 they reach the size of the parent individuals. A much commoner 

 mode of asexual reproduction among multicellular animals is that 

 of budding or gemmation where the body divides unequally. 

 The common fresh-water polyp or Hydra is a well-known example 

 of an animal in which budding occurs. There is at first a pouch- 

 like outgrowth of the body wall, and this is the starting-point of 

 the new offspring. As the bud increases in size the characteristic 

 tentacles and mouth of the young polyp make their appearance 

 and the other organs and structures are gradually formed. 

 Eventually the bud constricts at its base and breaks off from the 

 parent, so becoming an independent individual which goes on 

 growing till it reaches the size of the parent. Reproduction by 

 budding is very common among the lower groups of the animal 

 kingdom. The individuals produced in this way often remain 



