CHAPTER V 



CELLULAR DIFFERENTIATION. TISSUES 



64. Two things of importance happen to the body as the 

 organism develops from the simple condition of the ovum to the 

 great complexity of structure in the adult : (i) the increase in the 

 number of cells, which is quantitative in nature, and (2) the 

 differentiation of cells, whereby the cells of the various parts 

 become very diverse in shape, composition, and powers. This 

 is a qualitative change. It is not yet fully known how much 

 of the difference in the cells of the various tissues is due to 

 qualitative differences in the daughter cells of a given division, 

 and how much is due to external influences and the interrela- 

 tions of the cells after division. We know, for example, that 

 gravity acting on the food substance of the ovum before division 

 does produce such differences among the daughter cells of the 

 early cleavage stages as lead to results as diverse as ectoderm 

 and entoderm. Doubtless there are also internal processes that 

 tend to give rise to similar differences. On the other hand, it has 

 been shown by experiment that, even as high up in the animal 

 scale as the lowest vertebrates, the blastomeres of the two- or four- 

 celled stage may be shaken apart and each develop into a small 

 but perfect embryo. This experiment shows that up to this 

 stage no specialization has taken place which limits the products 

 that come from these cells. The blastomeres do not so develop 

 after the 8- or i6-celled stage is reached, so far as is known. 

 We are ignorant of the causes which determine that one cell 

 shall develop into a muscle cell and its neighbor into a bone 

 cell. 



65. Tissues. A tissue is to be defined as a group of similar 

 cells suited by their differentiation to the performance of a 

 definite function. This differentiation affects the size, shape, 

 and the interrelations of cells, and likewise the chemical and 



47 



