10 



GROWTH 



crease in size and the differentiation results in the development 

 from this many-celled sphere of the parts and organs with 

 which we are familiar in the mature individual. This is shown 

 in the series of drawings in Figures 5 and 6 which depicts the 

 differentiation observable without detailed microscopic study in 

 the growth of the chick. The drawings show none of the cells. 



During the first twenty-four 

 hours of incubation the young 

 chick has grown far beyond 

 the stage of growth compara- 

 ble with the last drawing of 

 the salamander in Figure 4. 

 To the unaided eye it appears 

 as nothing but a line on the 

 surface of the egg yolk ( 1 and 

 2). During the second day 

 (3) the head and heart have 

 formed. During the third day 

 (4 and 5) the eye and tail be- 

 come visible. The fourth day 

 (6) the beginnings of the 

 wing and leg are discernible. 

 These parts as well as others 

 become more prominent the 

 fifth day (7 and 8), the 

 seventh (9), and eighth day (10), until by the tenth day (Fig- 

 ure 6) differentiation has proceeded far enough to permit any- 

 one to recognize the birdlike characteristics of the creature. 



Associated with the differentiation or development of the 

 organs and parts of the body is a differentiation of the cells 

 which compose them. They do not retain their embryonic ap- 

 pearance, but change in shape and contents and become special- 

 ized in function. Compare the differentiated cells from a ma- 

 ture animal (Figure 7) with the isodiametric embryonic cell. 

 Differentiation, which is the name we give to the phase of 



Figure 6. A chick remcved from an egg 

 which had been incubated 10 days and 2 

 hrs. After Keibel. From Minot's Age, 

 Growth and Death. By permission of G. P. 

 Putnam's Sons. 



