THE FOUR LAWS OF AGE 223 



means that after a cell has progressed and is differen- 

 tiated a certain distance, its fate is by so much deter- 

 mined. It may from that pass on, turn in one direction 

 or another, always progressing, going onward in its 

 cytomorphosis ; but the general direction has been 

 prescribed, and the possibilities of that cell as it pro- 

 gresses in its development become more and more 

 restricted. For instance, the cells which are set apart 

 to form the central nervous system after they are so 

 set apart cannot form any other kind of tissue. 1 After 

 the nervous system is separated in the progress of 

 development from the rest of the body, its cells may 

 become either nerve cells proper or supporting cells 

 (neuroglia), which latter never acquire the nervous 

 character proper, but serve to uphold and keep in 

 place the true nervous elements. They represent the 

 skeleton of the central nervous system. After the 



" multiplum lebensfahiger und gestaltender Substanz." As the ovum can pro- 

 duce two ova, it can produce two individuals. Ectoderm cells can produce 

 more ectoderm, but not a whole new individual. 



i An unexpected exception to this statement has been discovered, which is, 

 however, more apparent than real. In certain Amphibia it has been found that, 

 if the lens of the eye is extirpated from a young larva, a new lens will be 

 formed at the expense of the retina, which itself arises from an outgrowth of 

 brain tissue. At the time the retinal lens is produced, however, the retinal cells 

 are still in an undifferentiated state, and those retinal cells which have advanced 

 to the stage of young differentiated elements cannot produce a lens. The 

 normal lens is developed from the outer skin (epidermis) of the embryo. See 

 A. Fischel, Anatomische Hefle, xiv., p. I, (1900) and Archiv. f. Entwickelungs- 

 mech., xv., p. i; G. Wolff, Archiv, f, Entwickelungsmech., i., pp. 380- 

 390, and xii., pp. 307-351 ; W. H. Lewis, American Journal of Anatomy, iii. t 

 505-536, 1904. It is important to note that the retinal lens differs greatly in 

 its cell structure from the normal lens, and is smaller, though resembling it in 

 general form. 



