INTRODUCTION 



time of its life a salamander looks like the drawing and consists 

 essentially of a nucleus, cytoplasm, and some food material. 

 These parts are not shown in the drawing because it represents 

 a surface view. The second drawing shows the beginning of the 

 formation of a two-celled sala- 

 mander which is completely 

 formed in the third drawing. 

 An eight-celled salamander is 

 shown in the fifth drawing. In 

 the ninth drawing the sala- 

 mander consists of cells too 

 numerous to count. The result 

 of these divisions is the pro- 

 duction of an ever increasing 

 number of cells arranged in a 

 sphere. Although the cells are 

 much alike some differentia- 

 tion has occurred, even in these 

 early stages of growth. The 

 cells differ in density of pro- 

 toplasm, in size or color, in 

 quantity of food material, and 

 particularly in their later his- 

 tory. Some cells which can be 

 recognized in these early 

 stages by their position rela- 

 tive to other cells and by their 

 contents form germ tissue, 

 some form nerve tissue, and so 

 on. The further growth of a vertebrate animal is too compli- 

 cated for us to consider in detail here. It may be sufficient to 

 say that the cells making up this sphere increase in number and 

 differentiate by a regular progression of stages to form the ma- 

 ture individual. The increase in the number of cells together 

 with the enlargement of the cells themselves results in an in- 



Figure 5. Ten stages in the growth of a 

 chick. Times of incubation are as follows: 

 No. 1—20 hrs., No. 2—24 hrs., No. 3—2 

 days, No. A — 2 days, 19 hrs., No. 5 — 2 

 days, 22 hrs., No. 6 — 3 days, 16 hrs., No. 

 7 — 4 days, 8 hrs., No. 8 — 5 days, 1 hr., No. 

 9 — 7 days, 4 hrs., No. 10 — 8 days, 1 hr. 

 After Franz Keibel. From Minot's Age, 

 Growth and Death, By permission of G. P. 

 Putnam's Sons. 



