12 PRINCIPLES OF EMBRYOLOGY 



region (such as the spinal column). The arising of differences between the 

 spatial parts of the zygote is, somewhat more commonly, spoken of as 

 'segregation' (or 'Sonderung' in German), words whose main drawback is 

 that they tend to suggest a particular mechanism for the process, namely a 

 sorting out into two separate positions of materials which were originally 

 mingled. The word 'regionahsation' is also used as another name for the 

 process, and is perhaps preferable, as being more neutral in its implications. 



The third basic type of process is the moulding of a mass of tissue (or, 

 in Protozoa, of a part of the cell) into a coherent structure which is recog- 

 nised as having some unitary character of its own, which is usually ack- 

 nowledged by giving it a name as an anatomical organ. Thus the neural 

 plate does not merely undergo histological differentiation and regionahsa- 

 tion, to give separate masses of forebrain tissue, midbrain tissue and hind- 

 brain tissue, but also becomes moulded into the characteristic shapes of 

 these organs. The forming of a mass of cells into a new shape is known as 

 'morphogenesis'. In the abstract, one can conceive of it as occurring quite 

 by itself, without any accompanying histological differentiation or 

 regionahsation. But it is only rarely, in simple organisms such as Myxo- 

 mycetes or in special situations such as cells growing in tissue culture, that 

 this happens. Much more usually, the morphogenesis of an organ is 

 accompanied by tissue differentiation and often by the appearance of 

 distinct spatial sub-units (regionahsation). For such complex processes, 

 when we wish to emphasise morphogenesis as the main component, the 

 name 'individuation' has been proposed. 



It will be realised, of course, that in the actual phenomena of embryonic 

 development, changes of all these three types are usually closely inter- 

 woven with one another. It is true that in experimental situations histo- 

 logical differentiation can occur with no regionahsation and very little, 

 if any, individuation. But regionahsation is nearly always accompanied 

 by some individuation, since the newly appearing regions are normally 

 related to one another in some definite pattern. And we have already 

 noticed that morphogenesis by itself is something of a rarity. Never- 

 theless it is important to disentangle them from each other, since each 

 requires a different category of explanation. Differentiation could be 

 a purely chemical process, involving nothing more than changes in 

 substances (which, however, might exist in larger particles than the 

 molecules of conventional chemistry, for instance in cell granules, mito- 

 chondria, etc.). Regionahsation, on the other hand, involves some refer- 

 ences to a spatial framework; it requires at least physico-chemical notions, 

 such as diffusion, crystallisation or the like. Finally, the moulding of a mass 

 of material into a shape, as in morphogenesis, can only be brought about 



