REPRODUCTION AND DEVELOPMENT 283 



out the developmental period, the embryo exhibits a region or re- 

 gions that lead in the appearance of new structures and seem to act 

 as dominant elements, controlling and coordinating the initiation 

 of developm.ental processes of other regions. In the gastrula the 

 dorsal lip of the blastopore is such a dominant element. If the 

 dorsal lip of an early gastrula of a salamander embryo be cut out 

 and transplanted to an abnormal position on another gastrula of 

 approximately the same stage of development, it in some way 

 causes the cells of the host in its vicinity to develop into neural 

 plate cells and eventually to form an abnormally situated neural 

 tube. This action of a bit of tissue foreign to the developing host is 

 rather amazing, for the ectodermal cells that otherwise would 

 normally develop into the superficial layer of the skin, under the 

 influence of the transplanted tissue become differentiated into nerve 

 tissue with the extremely specialized characters that distinguish that 

 tissue (Fig. 193). 



When the neural plate has formed, apparently under the influence 

 of the dorsal lip of the blastopore, the antero-posterior axis of the 

 new animal has been definitely established. The anterior end of 

 the neural plate and subsequently developing neural tube now ap- 

 pears to become the leading element; the blastopore either closes 

 or becomes differentiated into other structures. Development is now 

 marked by the fact that the anterior regions of the embryo lead in 

 undergoing the chemical and structural transformations of growth 

 and differentiation. Thus the features resembling the adult form of 

 the head are first to appear; the anterior limb buds form first (Fig. 

 184). The anterior end of the nervous system differentiates into the 

 parts of the embryonic brain and assumes an advance over more 

 posterior nervous tissue. This character of development is known 

 as the Law of Antero-posterior Development; briefly stated, the 

 law is, that, in general, in the development of the animal or of any 

 of its organ systems the most anterior region is first to appear. 

 These facts constitute visible evidence of an underlying principle of 



