70 VITAL ANIMAL LIFE PROCESSES 



either fission or budding, these methods are known as asexual reproduc- 

 tion — reproduction without sex. 



In most forms of animal life sexual reproduction is the only method. 

 Sexual reproduction involves a method of mingling the genes from 

 different cells and in most cases from different animals. Nearly all 

 animals with sexual reproduction produce eggs and sperms, each of 

 which carries approximately the same number of hereditary character- 

 istics, the genes. When the sperm fertilizes the egg, there is a mingling 

 of genes from two different individuals and this produces a variety in 

 the offspring which could not be possible with asexual reproduction. 

 In some of the one-celled organisms, two animals will come together 

 and trade genes in a form of sexual reproduction known as conjugation. 

 This achieves the variety that comes with the mingling of genes even 

 though no sperms or eggs are produced. There are many of the simpler 

 animals which have both sexual and asexual reproduction. Only in a 

 few of the one-celled animals does asexual reproduction take place ex- 

 clusively. The biological importance of this random assortment of the 

 genes in sexual reproduction will be brought out as we continue our 

 study. 



Tissues, Organs, and Organ Systems 



In the small animals with a simple body organization, the cells are 

 somewhat alike all over the body; but as animals become larger there 

 is a gradual increase in specialization — some cells are formed which 

 perform certain specific functions most efficiently whereas other cells 

 are specialized in different ways for different functions. In such ani- 

 mals the cells are organized into tissues. A tissue is defined as a 

 group of similar cells performing a particular function. Tissues may 

 secrete intercellular materials as in the case of bone tissue, which con- 

 sists of bone cells surrounded by a hard matrix containing lime salts. 

 Blood is a tissue which consists of cells together with a fluid matrix 

 in which the cells, called corpuscles, float. Your outer layer of skin, 

 the epidermis, consists of epithelial cells with a very thin layer of sticky 

 secretion which holds them together, yet allows them to rearrange them- 

 selves. We shall study tissues in more detail when we study the struc- 

 ture of the vertebrate animal. 



Different kinds of tissues combine to form organs, such as the 

 stomach, kidney, and thyroid gland. The stomach, for example, con- 

 sists of four distinct layers, each composed of different kinds of tissues ; 

 and the combined action of these tissues is necessary if the stomach 

 is to perform its specific functions in the role of digestion. 



