44 The Development of a Primifive Animal 



protuberance and ultimately a complete limb. We can ask, therefore, why 

 it is that the limb forms where it does and why other areas that have the 

 potency for limb construction normally do not grow one. 



A system in which many parts can develop in a certain way but 

 where only one part actually does so is called a morpho genetic field, 

 which will be discussed in detail in the next section. The dominance of 

 one part of the field over another maintains regularity of development. 

 It ensures that a coelenterate polyp will have one head and that it will be 

 where it ought to be, i.e., at the top of the stem. It ensures that a chicken 

 wiU have one tail, two wings, and two hind limbs in the right places. 

 Moreover, it makes for versatility, for if by accident the dominant area of 

 a morphogenetic field is destroyed, a neighboring area has the potency to 

 take its place. Thus the top of the coelenterate stem and only the top 

 normally forms a head. But if by accident the head is destroyed, a new 

 one can be constructed from underlying tissues. 



Morphogenetic Fields 



The concept of a morphogenetic field had its modern inception in 

 the work of Professor C. M. Child, a famous American embryologist. Dr. 

 Child was the first to emphasize the fact that in such fields one area will 

 gain dominance over another by its superior capacity to form a given 

 structure and by its ability to inhibit neighboring areas from doing so. He 

 and his students provided many examples of field development in the 

 regeneration of coelenterates, flatworms, and starfish and in the forma- 

 tion of embryos. Thus we speak of head fields, limb fields, tail fields, heart 

 fields, etc., to denote this kind of development. Attempts have been made 

 to describe morphogenetic fields in mathematical terms in an effort to 

 understand the processes by which dominance is established. A simplified 

 version of the mathematical argument is given below. 



Consider a group of coelenterate stem cells that can form a head. 

 We divide this group into two separate areas. The speed at which the 

 stem cells in an area will construct a head should depend on their inherent 

 capacity (i.e., developmental potential) to be converted into head cells 

 and also on the number of cells available in the area for head construction. 

 This can be written formally as: 



rate of head development = (developmental potential) 

 X (number of stem cells) 



The rate of development in area I might be greater, equal to, or less than 

 that of area II, depending on the relative developmental potential and on 

 the numbers of cells that comprise each area. However, a difference in 

 developmental potency is not enough to create a morphogenetic field, 



