134 GENERAL CONCEPTS 



Epidermis-] pMearal 



Liiie of 

 inva^inatic 



EctoderiTT.- 



Mesoderm. 

 Endoderni 



rNeui^al area. 

 ■Wotocliord 



mesodenn 



A 



dorsal lip 

 blastopore- 



B 



Somite 

 mesoderm 



iTiesoderm 



Figure 6.11. Embryo maps. A, Lateral view of a frog gastrula showing the pre- 

 sumpti\e fates of its se\eral regions. H, Top \iew of a chick embryo showing location in 

 the primitive streak stage of the cells which will form particular structures of the adult. 



that certain potentialities are localized at an early stage, for neither part 

 can develop into a whole embryo. Each halt develops only those struc- 

 tures it would have formed normally as part of the whole embryo. This 

 localization of potentialities eventually occurs in the development of all 

 eggs; it simply occurs at an earlier stage in some species than in others. 

 It has been possible by experimental techniques to map out the areas of 

 potentialities in the early amphibian gastrula and in the primitive streak 

 stage of the chick (Fig. 6.11). 



In the past, biologists have speculated that differentiation might 

 occur (1) by some sort of segregation of properties during mitosis, (2) 

 by the establishment of chemical gradients within the developing em- 

 bryo, (3) by somatic mutations, or (4) by the action of chemical organ- 

 izers. Recently the induction of adaptive enzymes in bacteria has been 

 used as a model system to provide another explanation for embryonic 

 differentiation. Experiments have shown that bacteria (and, to some 

 extent, animals as well) can respond to the presence of some new sub- 

 strate molecule by forming enzymes which will metabolize it. Jacques 

 Monod, of the University of Paris, has suggested that in an analogous 

 fashion, extracellular or intracellular influences may initiate or suppress 

 the synthesis of specific enzymes, thus affecting the chemical constitution 

 of the cell and leading to differentiation. The enzyme complement of a 

 cell is, to some extent, plastic, and can be changed by extra- or intra- 

 cellular influences. As an embryo develops, the gradients established as a 

 result of growth and cell multiplication could result in quantitative and 

 even qualitative differences in enzymes. As a result of the stimulation or 

 inhibition of one enzyme, a chemical product could accumulate which 

 would induce the synthesis of a new enzyme and thus confer a new 

 functional activity on these cells. 



Morphogenesis is probably too complicated a phenomenon to be 

 explained in terms of a single phenomenon such as enzyme induction. 

 Enzymes can indeed be induced in an embryo by the injection of a suit- 

 able substance. Adenosine deaminase, for example, has been induced by 



