30 GENERAL CONCEPTIONS. 



protoplasm, but particles imbedded in the protoplasm or, as they may be called, 

 enclosures. 



The Law of Genetic Restriction. — Another fundamental idea, which it is 

 most important for the student to grasp, is that differentiation acts as a pro- 

 gressive restriction upon the further development. Each successive stage of 

 differentiation puts a narrower limitation upon the possibility of further advance. 



The range of possible changes at any given time is determined not merely 

 by the nature or kind, but also by the stage or degree of the previous differentia- 

 tion. The law of genetic restriction dominates the entire ontogeny. In order to 

 illustrate it and to emphasize it, it will be profitable to consider a few illustra- 

 tions from each of the germ-layers. First, then, the ectoderm. This layer early 

 separates into two parts, one to form the nervous system, the second the epider- 

 mis; the nervous part thereafter never forms epidermal structures, the epider- 

 mal part never forms a medullary canal. The central nervous system retains in 

 part a simple epithelial character (ependyma proper), but most of its walls 

 become nervous tissue; its cells pass from the indifferent stage and become neu- 

 roglia cells or young nerve-cells (neuroblasts). Neuroglia cells never become 

 anything else, and the nerve-cells are always nerve-cells to the end. Next, as 

 to the entoderm. Wherever in it specialization takes place, as in the tonsil, 

 thymus, thyroid, oesophagus, liver, or pancreas, each territory of cells keeps 

 its characteristics and never assumes those of another territory. Finally, as 

 to the mesoderm. It is found very early to include in vertebrate embryos 

 four kinds of cells, of which the most numerous are the undifferentiated cells, 

 the other three kinds being the endothelium of blood-vessels, red blood-cells, 

 and germ-cells. All of these are precociously specialized; they are few in 

 number, yet they are probably the parents of all the cells which are produced of 

 their kind each throughout life. Passing on to a later stage, we note that 

 when a striated muscle-fiber is produced a striated muscle-fiber it always re- 

 mains, and it never becomes anything else. 



Two Types of Differentiation. — There are two distinct types of cell differen- 

 tiation which I think have not hitherto been clearly recognized or defined. For 

 both types the starting-point is the same — the undifferentiated embryonic cell. 

 In one type we find that, as the cells proliferate, a portion of them only under- 

 goes differentiation, and another portion remains more or less undifferentiated 

 and retains more or less fully the power of continued proliferation. The epi- 

 dermis is a good representative of this type. Its basal layer consists of embryo- 

 nic cells, which multiply; some of these cells move into the upper layers, enlarge, 

 and differentiate themselves into horny cells; others remain in the basal layer 

 and continue to multiply. The progeny of a given basal epidermal cell do not 



