CHAPTER V. 

 CELLULAR DIFFERENTIATION. TISSUES. 



61. Two things of importance happen as the organism de- 

 velops 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 that gravity act- 

 ing 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. On the other hand, it has been shown by ex- 

 periment that, even as high up in the animal scale as the 

 lower 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 de- 

 velop 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. 



62. 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 

 physical structure of the protoplasm, in such a manner as to 



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