THE HEREDITARY FACTORS AND DIFFERENTIATION 403 



with a specific normal environment.^ An equally good case is that 

 of the adult characters of the axolotl. As is well known, the adult 

 characters (the genes controlling which have been inherited for 

 countless generations) normally fail to appear, as the animal is almost 

 invariably neotenous and does not undergo metamorphosis. But 

 spontaneous metamorphosis does occasionally occur under certain 

 conditions of the external and internal environment ; in particular, 

 the administration of thyroid hormone. In the absence of these 

 environmental conditions, the genes are powerless to produce the 

 adult characters. 



§3 



While the genes are by themselves incapable of initiating the pro- 

 cesses of development and differentiation, it is obvious that they 

 play an active part in the control of these processes, once develop- 

 ment has been started, and their presence is essential. A good 

 illustration of this is provided by sea-urchin eggs when fertilised by 

 two sperms. Each sperm brings with it an aster which divides, with 

 the result that there are four, and a quadripolar spindle may be 

 formed in the egg. Such an egg contains three nuclei, and since 

 each is haploid, there will be three 7i chromosomes spread at 

 random over the four spindles. Each chromosome divides, thus 

 producing six ?i chromosomes in all, to be distributed between the 

 four blastomeres into which the egg divides at once. On the 

 average, therefore, there will be 6/Z/4, or 1*5?/, chromosomes to 

 each blastomere. 



It is known from experiments on parthenogenesis that the hap- 

 loid number of chromosomes, or n, is sufficient to enable develop- 

 ment to occur, and therefore, if all the chromosomes were equiva- 

 lent, any blastomere which received at least n chromosomes might 

 be expected to develop. But such is not the case. If, on the other 

 hand, it is assumed that each chromosome of each genome is 

 functionally different, so that when a particular chromosome is 

 absent its place cannot be taken by any other chromosome of the 

 same genome, but it can be supplanted by the corresponding chromo- 

 some of one of the other genomes, then it is possible to calculate the 

 chances in favour of any one blastomere receiving at least one 



^ Goodrich, 1924, p. 56. 



26-2 



