80 



EMBRYOLOGY 



possess some organization itself? In addition, the concept implies an organ- 

 izing tissue which possesses some organization itself and also the ability to 

 organize other tissues into nervous structures. Another way of expressing the 

 relationship is to say that the region around the dorsal lip is dominant over 

 the rest of the egg or exerts a control over its development. 



Comparison of amphibian gastrula 

 and chick primitive streak 



Having outlined the principles of early embryonic development in the 

 amphibian egg, let us see how they apply to the hen's egg. The presumptive 

 values of the early chick gastrula, called a blastoderm, have been mapped in 

 much the same manner as those of the amphibian egg. In the chick it is the 

 primitive streak, a thickening in the blastoderm (Fig. 37), which serves as a 

 marker of polarity. The anterior end of the primitive streak, for reasons 

 which we shall consider in Chapter 15, is homologous with the dorsal lip of 

 the blastopore of the amphibian gastrula. That is, the presumptive notochord 

 and mesoderm are located around the anterior end of the primitive streak, 

 just as they are found above and to the sides of the dorsal lip of the 

 amphibian blastopore. 



The primitive streak is not only homologous with the lips of the blastopore 

 but it can also be shown to have the same organizer properties. These prop- 

 erties can be demonstrated by some simple transplantations (Fig. 38). If the 



NOTOCHORD 

 MESODERM 



AMPHIBIAN GASTRULA CHICK PRIMITIVE STREAK 



Fig. 37. Comparison between the amphibian gastrula and the chick blasto- 

 derm. Arrows show the position of and direction of migration of the notochord 

 and mesoderm. On the basis of homology, we should expect the organizer in 

 the chick blastoderm to be located at the anterior end of the primitive streak. 



