CHAPTER 6 



Genes and Development^ 



The mode of action of genes during development can be investigated 

 in two ways: (i) by experiments on the mechanism of development, 

 (2) by examination of the changes produced in developing organisms 

 by gene-changes. The first field of inquiry is usually considered to 

 constitute the separate science of experimental embryology, while the 

 second is a part of genetics, and is often referred to as phaenogenetics. 

 Such a separation is, however, very artificial; both subjects fall into the 

 general field of investigation of how an adult organism arises from the 

 individuals of the previous generation. Thus some at least of the 

 results of experimental embryology are essential for a full understanding 

 of genetics, and in this chapter we shall consider the general mechanism 

 of the development of animals and the ways in which genes may act to 

 control the course of the reactions involved. Unfortunately there are 

 no adequate data for a similar survey of genetic control of plant 

 development. 



I. The Equal Division of the Nucleus 



The simplest hypothesis to account for the genetic control of develop- 

 ment might seem to be that the genes were gradually sorted out by 

 unequal divisions of the zygote nucleus, so that each part of the embryo 

 received a different collection of genes, which could then determine the 

 partioilar way in which that part should develop. Actually the hypo- 

 thesis is not so simple as it seems; it would be difficult to imagine a 

 mechanism for the differential divisions without assuming that the 

 kind of genes which went into a particular nucleus was determined 

 by the kind of cytoplasm surrounding the nucleus; without, in fact, 

 assuming exactly that differentiation of the embryonic regions which 

 the hypothesis was invented to explain. However, the facts spare us 

 the necessity of devoting further thought to the hypothesis. It is 

 untrue. There is evidence from many different groups of animals that 

 all the first cleavage nuclei, at any rate, are equivalent and can be 

 substituted for one another."^ The first steps in differentiation therefore 



^ General references: Daleq 1935, 1938, Huxley and de Beer 1934, Morgan 1927, 

 1934, Schleip 1929, Spemann 1938, Weiss 1930. ^ /^^^^ Morgan 1927. 



