History of the Theory of Heredity. 23 



each other, but like the constituant cells of all parts of 

 the body of the organism. 



Far from being preformed in the egg, we know that 

 the body is built up gradually, step by step, by repeated 

 cell-division, and that the earlier stages of development 

 do not result in the formation of the parts of the perfect 

 body at all, but that they simply give rise to germ-layers, 

 or tracts of cells out of which organs are gradually 

 formed, and that cells which were at first quite widely 

 separated in the embryo may come at last to enter into 

 the formation of a single organ. 



For instance, when the nervous system of a vertebrate 

 first makes its appearance in the embryo, there are no 

 traces of the brain, of the spinal cord, of the nerves or 

 of sense organs. It at first consists of a long group of 

 cells running along the middle line of the body, and 

 presenting no difference from the other cells of its 

 surface. In most cases this elongated group of cells 

 becomes converted into a furrow, and afterwards into a 

 closed 'tube, the nerve-tube, by the folding together of 

 its edges. The primitive nerve tube is at first simply a 

 long tube of embryonic cells running along the middle 

 line of the back,- and it is a very different thing from the 

 final nervous system of an adult mammal, nor is it in 

 any sense a mammalian nervous system in miniature, for 

 the changes by which it becomes converted into the lat- 

 ter are great and numerous, as well as gradual. Certain 

 parts, suclr as the eye, are formed only in part from this 

 tract of cells, for the vertebrate eye is the result of the 

 combination of an outgrowth from the embryonic ner- 

 vous svstem and an ingrowth from the surface of the head. 



The whole history of the nervous system and sense 

 organs is thus seen to directly oppose the view that these 

 organs are present in miniature in the germ. 



