DIFFERENTIATION 679 



fish of the genus Periophthalmus, an animal of the East Indies which 

 comes out of the sea and walks on land, breathing through its 

 tail, causes them to exaggerate their terrestrial adaptations, and 

 the pectoral fin comes to resemble a pentadactyl limb. It looks 

 as if the fish had the potentiality of becoming a tetrapod, which 

 only needed liberating just as the organiser liberates the sub- 

 stance in amphibian blastula cells which enables them to form a 

 nerve cord. The differential development of the kidney ducts in 

 male and female vertebrates depends on hormones from the testis ; 

 the pronephric duct becomes an oviduct unless it is suppressed, 

 while the mesonephric duct forms a Wolffian duct only if male 

 hormone is present. 



Lastly, much of the detailed development of an organ or a tissue 

 depends on its continued use. Muscles become bigger when they 

 are continually stimulated to contract, skeletal tissues grow 

 where the body is most subject to strain, and the blood vessels 

 in the chick enlarge in proportion to the speed at which blood 

 flows through them. 



There seem then to be six influences which determine the form 

 and function which any cell will finally have, (i) The genes 

 which it inherits from its parents. Rarely, as in the red blood 

 cells of mammals and in epithelia where division of the nucleus 

 is amitotic (p. 694) these, having done their work, may abdicate 

 from any further influence. It seems that many of the adult 

 tissues of mammals have different numbers of chromosomes ; 

 if this is so part of their differention might be due to the different 

 genes which they receive. (2) The external environment, which 

 co-operates with the genes. (3) The position of the egg in the ovary, 

 or relative to the sperm at fertilisation, which determines uneven 

 distribution of substances within the cytoplasm. (4) The position 

 of some cells relative to others, which by virtue of their special 

 contents are dominant and impose a pattern on their neighbours. 

 (5) Hormones liberated by special parts of the body, and (6) use 

 and disuse. Each of these acts on a partial differentiation produced 

 by its predecessors in the hierarchy, and the differentiation is 

 progressively and rapidly increased. 



