THE SCIENCE OF EMBRYOLOGY II 



involve interference w^ith embryos by surgical means, or by treatment 

 w^ith chemical or physical agents and so on ; and those of developmental 

 genetics, in which the embryos are 'experimented on' by controlling the 

 genetic constitution of the gametes from v^hich they arise. These two lines 

 of approach have led to two bodies of knowledge which are as yet only 

 somewhat imperfectly brought into relation with each other, and in the 

 outline given below it will be convenient to treat them separately. An- 

 other point to which attention should be drav^oi is the fact that the attempt 

 to understand a previously unknown causal system is nearly always a slow 

 process. In most causal sciences, and certainly in causal embryology, a long 

 period of investigation is necessarily devoted to discovering the general 

 nature of the causal systems involved, and only after this endeavour has 

 made considerable progress is it possible to get down to the concrete 

 details of how the various mechanisms work. As will be shown below, 

 most of the theories of experimental embryology do not attempt to do 

 more than describe the kind of system which is operating ; it is only in 

 quite recent times, and then only in a few instances, that one can begin 

 to envisage the specific chemical reactions or physical forces concerned. 



There are three basic types of phenomena which occur during embry- 

 onic development, and for which a causal science has to attempt to find 

 some explanation. The first is the gradual change in the nature of a mass 

 of living matter, which may consist of a part of a cell or more usually of 

 a group of many cells. For instance, we see the coluronar epithelial cells 

 of the early neural plate gradually assume the characteristic appearance of 

 the central nervous system, with its elaborate arrangements of nerve 

 fibres ; or the roughly cuboidal cells of the somites become elongated and 

 fdled with myosin until they are recognisable as muscle fibres. Such 

 phenomena may be called 'histological differentiation'; and it is most 

 correct, indeed, to reserve the word 'differentiation' for changes of this 

 kind even when it is used without qualification. 



A second type of phenomenon is the arising of differences between the 

 various parts of the embryo. Soon after fertihsation we may be able to 

 recognise only two or three different regions, while at a later stage there 

 will be many more individually characterised organs. Again within any 

 one organ, such as the neural system, there are at first only a few distinct 

 sub-units, in contrast to the numerous parts into which it becomes 

 naturally divided later on (the fore-, mid-, and hind-brain, the spinal 

 column, etc.). This phenomenon might be referred to by the expression 

 'regional differentiation', but actually that is usually, and better, employed 

 to indicate the type of histological differentiation characteristic of one 

 region (say the ^orebrain) when it is contrasted with that of some other 



