GROWTH AND REPRODUCTION 45 



becoming constantly less rapid, until the maximum size is attained, which 

 is usually somewhat beyond middle life. There is then a slow decrease 

 in weight. Throughout the rest of the period of maturity this decrease 

 continues, becoming gradually more pronounced; after the individual 

 passes into old age, however, there is a more rapid emaciation, which 

 ends with death. The youthful condition is termed adolescence; that 

 accompanying old age, senescence. 



71. Limit of Size. — No matter what the character of the life cycle 

 may be or when growth takes place or for how long, there is in the case of 

 all animals a size Hmit which is not surpassed. While in one-celled 

 animals this varies somewhat in different lines of descent, in any one 

 line it is rather closely approximated. In such forms there seem to be 

 some metabolic relations in the cells, which, as this hmit is approached, 

 give rise to changes which automatically result in the division of the cell 

 and the production of smaller organisms. The same thing is true of the 

 individual cells which compose the bodies of higher animals, but the 

 result is to produce a larger body and not new individuals. In some 

 higher forms the process of cell multiplication practically ceases as the 

 individual becomes adult; in other forms it stops in certain parts of the 

 body but goes on in other parts. The organs of the central nervous 

 system reach full size early in life. Bones and muscles continue to grow 

 until the animal becomes adult. The skin, however, from the outer 

 layer of which dead cells are continually shed, grows throughout Hfe by 

 the multiplication of hving cells in the deeper layers. The size of many- 

 celled animals is also limited by various factors such as inheritance, the 

 available food, and the activity of glands the secretions of which favor 

 or hinder growth. 



72. Reproduction. — Multiplication by cell division, which is the 

 most common way among one-celled animals, is not possible to those 

 which are many-celled, since the different cells which make up the latter 

 became varied in form and structure and also become specialized for the 

 performance of one or a few functions out of the many that are possessed 

 by the body as a whole. Consequently such cells cannot reproduce the 

 complete animal. Under these conditions certain generaUzed or unspe- 

 ciahzed cells are set aside for the purpose of reproduction and are reheved 

 from the performance of any other duty. They serve as cells from which 

 the development of another individual may be initiated, transmitting to 

 that individual the characters of the parents in the bodies of which they 

 have been produced. Such cells are termed sex cells or gametes. Of these 

 there are two types which in the higher animals are known as egg cells 

 and sperm cells, or sperms. The animal which produces egg cells is called 

 female and the one which produces sperm cells, male. Generally speak- 

 ing, egg cells are relatively large in size and sperm cells relatively very 

 small, so that the former may be termed macrogametes and the latter. 



