212 



BIOLOGY AND HUMAN LIFE 



supply, and the removal of wastes— all are unified by this 

 transportation system. 



In addition to the main sets of organs already mentioned 

 there are in the body several glands, some of them paired, 

 which throw their special fluids directly into the blood (see sec- 

 tion 145). These ductless glands are sensitive to very slight 

 changes in the chemical condition of the blood, and in turn 

 the substances which they discharge into the blood produce 

 striking effects upon the protoplasm in all parts of the body. 



Fig. 107. Early stages in the development of a frog 



The development of the frog's egg may represent for us the development in all back- 

 boned animals. The fertilized egg, a, divides into two cells, b. Each of these divides 

 again, c, and the process is continued. As the number of cells increases, there soon 

 begins a differentiation; that is, instead of the cells' continuing to be alike, some 

 become smaller, and in time distinct regions, organs, and tissues are distinguishable 



Through these internal secretions the strains and needs of vari- 

 ous organs are counteracted or supplied, so that the unity of the 

 organism is increased. 



Finally, the irritability of protoplasm manifests itself in more 

 developed animals by the formation of the nervous system. This 

 reaches all parts of the body and is sensitive to changes inside, 

 as well as to the changes and disturbances in the environment. 

 The nerves are connected not merely with the muscles and the 

 organs of special sensation (eye, ear, tongue, etc.) but also with 

 the blood vessels and with the ductless glands. Because of their 

 extreme sensitiveness and their quick response they constitute a 

 very striking system of coordination, or unification, in the body. 



