FACTORS IN EMBRYOGENESIS 11 



Thus we may ask : In the developing embryo, how do particular genes 

 become activated at the proper time so that the characteristic orderly 

 development ensues? Haldane (1932) has pointed out that any gene, or 

 other body involved in the transmission of hereditary characters, may 

 affect any of six distinct stages in an organism: (i) the gamete carrying 

 it; (ii) the zygote carrying it; (iii) the endosperm carrying it; (iv) the 

 mother of the zygote carrying it ; the former not necessarily carrying 

 it; (v) a gamete (not necessarily carrying it) formed by a zygote 

 carrying it ; and (vi) a zygote (not necessarily carrying it) produced by 

 a mother carrying it. Different genes may come into action at different 

 times. Wettstein, for example, was able to observe the effect of one of 

 four genes in Funaria hygrometrica in the protonemal stage, whereas 

 the effects of the others did not appear until later. 



The embryos of related organisms tend to be more alike than the 

 adults, and this Haldane explains saying that the genes which determine 

 the interspecific differences exercise their chief effect, or come into 

 action, rather late in the individual development. But while this may 

 be so, it should also be pointed out that, as the organism grows to 

 large size, many other factors, not necessarily gene-controlled, may 

 enter into the situation. A common tendency, recognised by students 

 of evolution, is for certain characters to make their appearance pro- 

 gressively earlier in the life-cycle. In such instances, organogenesis is 

 said to be accelerated. This may be explained genetically by saying that 

 the first action of particular genes, which normally come into action at 

 a certain stage of development, has taken place at an earlier stage. 

 Again, organs present in an adult ancestor may be reduced to vestiges 

 and appear in the embryo of later evolutionary types. In the genetical 

 explanation, this may often be attributed to acceleration as defined 

 above. The interest of these views for the student of embryology needs 

 no emphasis. 



Another tendency, known as neoteny or paedogenesis, to which 

 Haldane directs attention, consists in a deferment of the action of 

 certain genes, with the result that originally embryonic characters may 

 persist in the adult. Neoteny is, in fact, the opposite of acceleration. 

 The result of neoteny is 'the sudden appearance in an adult form of new 

 characters, which have evolved in ancestral embryos.' De Beer (1930) 

 has described this process, which may have a wide application, as 

 'clandestine evolution.' In Haldane's view the possibihties of clandes- 

 tine evolution have probably been underestimated. The same process 

 may simultaneously accelerate or retard the action of a large number 

 of genes. Thus a change in permeability, due to a single gene, would 

 affect the time of action of many genes, with consequential effects on 

 development. Modifications of the time of action of a number of genes 



