=Degrees in cell differentiation and body organization.=--In the study of the cellular constitution of the tissues and organs of the toad, we found to what a high degree the differentiation of the cells may attain, and in the study of the anatomy of the toad we found how thoroughly these differentiated cells may be combined and organized into body-parts or organs. The body of the toad is made up of distinct organs, each composed of highly differentiated or specialized cells. The body of _Hydra_ is composed of cells for the most part only slightly differentiated and hardly recognizably grouped or combined into organs. These two conditions are the extremes in the body-structure of the many-celled animals. Between them is a host of intermediate conditions of cell differentiation and body organization. When we come to the study of other members of the great branch of simple many-celled animals to which _Hydra_ belongs (see Chapter XVII), it will be found that some of them show a slight advance in complexity beyond _Hydra_. Higher in the scale of animal life the forms will be found still more and more complex, with ever-increasing differentiation of the cells, with the combination of the differentiated cells into distinct organs, and the co-ordination of organs into systems of organs up to the extreme shown by the birds and mammals. And hand in hand with this increasing complexity of structure goes ever-increasing complexity or specialization of function. Breathing is a simple function or process with _Hydra_, where each body-cell takes up oxygen for itself, but it is a complex business with the toad, or with a bird or mammal, where certain complex structures, the lungs and accessory parts, and the heart, blood-vessels and blood all work together to distribute oxygen to all parts of the body. CHAPTER XII DEVELOPMENT OF THE TOAD FIELD AND LABORATORY EXERCISE