32 EVOLUTION AND GENETICS 



however specialized, cells are made up of two fundamental parts, 

 the nucleus and cytoplasm, the former embedded in the latter. 

 Each is complex, as will be seen in the diagram, and the two are 

 essential to each other. The nucleus appears to exert a controlling 

 influence over the cytoplasm, while the differentiation of the cyto- 

 plasm determines the chief characteristics of the various types of 

 cells. Some cells, such as the red blood corpuscles of most mam- 

 mals, are without nuclei, but after the loss of the nucleus their 

 lives are short and replacement occurs frequently. 



Plan of Animal Structure. Single-Celled Organisms. Although 

 it is necessary to call on our knowledge of individual development 

 for accurate interpretation of the conditions found in many 

 organisms, many others show a simple plan of structure; for ex- 

 ample, the fact that living matter cannot exist in units less com- 

 plete than the cell leads at once to the conclusion that single-celled 

 plants and animals are the most simple of all organisms (Fig. 14). 

 For that reason, if all life has really come from such a lowly origin, 

 they must represent the oldest forms now extant. They have 

 existed longer than any other forms, and have had opportunities to 

 become fitted for life under various conditions; hence we find many 

 species differentiated in various ways. Some are parasites in man 

 and other animals. If man is of recent origin, as we suppose, or 

 even if his origin followed that of the other animals, as all admit, 

 this is in itself evidence that the Protozoa have departed from 

 their original condition to become fitted for life in the other bodies; 

 they have evolved. 



The Germ Layers. During the development of many multi- 

 cellular animals a definite plan is followed in which the first step 

 is the repeated subdivision of the original germ cell to form a 

 hollow sphere, in the simplest state. This hollow sphere caves in 

 on one side until the two halves are in contact, thus forming a sac 

 with two layers of cells in its wall, called the gastrula. The inner 

 layer is associated primarily with nutrition and respiration, the 

 outer with the nervous system, sense organs, and protective skin. 

 Lastly, a third layer or mass of cells forms between the two, from 

 which develop the muscular and skeletal systems, circulatory 

 system, excretory system, reproductive system, and many sup- 

 porting and accessory parts of other structures. These three 

 layers of cells are called the germ layers. A group of similar cells 

 is obviously simpler than a sac with a two-layered wall of which 



