350 PRINCIPLES OF EMBRYOLOGY 



produced by minor abnormalities such as slight losses of material or the 

 substitution of one recessive gene for a dominant one. 



It is now necessary to discuss in rather more detail the successive phases 

 in this system of reactions. We shall leave till a later chapter the questions 

 relating to morphogenesis in the strict sense, that is to say, the mould- 

 ing of the developing tissues into defmite shapes. Here we shall be con- 

 cerned with the processes of chemical change during differentiation. 

 This is one of the most active, important and controversial fields of biology 

 at the present time. There are many different theories which wiU need to 

 be considered; and since the problems get so far down to the common 

 root of aU biological phenomena, light may be thrown on them from 

 very many different angles. We shall have to consider factual material 

 drawn not only from experimental embryology of the old-fashioned 

 kind, but from biocheinical studies and from the genetics of micro- 

 organisms as well as higher forms. If is often difficult to assess the relative 

 importance of these various types of evidence. On the one side we may 

 have what seems very precise and defmite biochemical information, 

 expressed in terms of nucleic acids, enzymes, phosphate bonds and so on — 

 but we have to ask ourselves just what is the connection between this and 

 the phenomena of development which we are trying to explain, and con- 

 sider whether it really is more enlightening than theories which operate 

 with less clear-cut concepts (ranging all the way from genes to organisers 

 and ooplasms) which are further from cheinistry but nearer to the em- 

 bryos. 



The fourth point mentioned above may be dealt with first. It is a 

 platitude, but an important one, that the body of a multicellular animal 

 is made up of tissues which are rather distinct from one another. Even 

 when changes in the normal course of tissue differentiation are brought 

 about (e.g. by induction, or the action of lithium on the amphibian 

 mesoderm, etc.) the altered tissue is usually switched from one into another 

 of the well recognisable types — from epidermis to neural tissue, or chorda 

 to somite. Intermediate types occur rarely, and when they do (as for 

 instance the 'palisade tissue' found as a 'weak' reaction to a neural-inducing 

 stimulus) they often later develop into something more normal, such as 

 ganglion tissue in the case of palisades. The developmental reactions, 

 therefore, tend to follow one or another of a number of definite paths, 

 which lead to rather well defined and distinct end-states. Further, there 

 is abundant evidence from all the regulatory phenomena which are so 

 common during development, that even if the conditions in a developing 

 tissue are made somewhat abnormal, the epigenetic system is often able 

 to compensate for this, so that the normal end-state is nevertheless 



