10 • INTERRELATIONSHIPS OF 

 DEVELOPING STRUCTURES 



Ihus 



hus far we have derived from the single egg an embryo composed of a 

 number of determined areas or fields which represent the individual struc- 

 tures and we have shown how these fields finally differentiate into formed 

 structures. The developing structures, however, are not isolated from each 

 other. A structure such as the limb needs nerves and blood vessels which 

 must come into it from other regions of the embryo. The question before us 

 now is how various tissues interact to produce a functional structure. 



Origin and development 

 of the neuroblasts 



We shall use the relationship of the developing nervous system to the 

 developing limb as the chief example. The problem can be summarized by 

 outlining the relationship of the primitive nerve cells — the neuroblasts — of 

 the early embryo to developing structures such as the limb. 



The source of neuroblasts in the developing embryo is, first of all, the 

 neural tube (Fig. 82). The neural tube contains large, rounded neuroblasts, 

 which will form the actual nerve cells and nerve fibers. Neuroblasts are also 

 found in the neural crests and the cranial ganglia. Some of the neuroblasts 

 from the neural crests migrate ventrally and form a depot of the neuroblasts 

 which give rise to the sympathetic ganglia. Thus we have another source of 



