CELLULAR DIFFERENTIATION 



57 



PIG. 27. Blood corpuscles (amphibian), c, colored corpuscles, flatwise and in profile; I, colorless 



corpuscles (leucocytes). 



FIG. 28. 



FIG. 28. 



Blood corpuscles (human), c, colored; I, leucocytes. 

 rows with the sides in contact. 



The red cells tend to collect in 



Questions on figures 27 and 28. Compare by means of the figures, the text 

 and reference books the colored and colorless corpuscles of these two types of 

 vertebrates and note the differences. In what other respects do the colored cells 

 differ from the white? Which are the less highly differentiated? Reasons for 

 your view? Why are the colorless corpuscles also called phagocytes? 



Dentine and enamel, though differing in structure from bone, are to be looked 

 upon as belonging to the same class of tissues. They differ chiefly in the fact that 

 no cellular elements are included in the secretion. They are thus harder and denser 

 than bone. 



79. We find all stages of transition between the more sim- 

 ple and more complex supportive tissues, and it may be seen 

 furthermore that there is a fundamental embryological sequence. 

 In the development of the organism the simpler connective tis- 

 sues give place, by transformation or substitution, to the more 

 complex. The cellular connective tissue of early life is replaced, 

 for example, by cartilage, and this may be transformed into 

 bone in adult life. 



