REPRODUCTION AND DEVELOPMENT IN CHORDATES 



Localizations occur in difTerent regions at the same time, but in a very 

 orderly manner. The same degree of localization will be found in any 

 vertebrate embryo of a given species at a given age (Fig. 5.22). Under 

 normal environmental conditions developmental processes occur with machine- 

 like precision. 



Gill slits 



Somites 



Lower jaw 



Ear vesicle 



Naris 



Umbilical 

 cord 



Tail 



Pig 



Man 



Fig. 5.22. Vertebrate embryos of comparable age. (Redrawn from B. M. Patten, Early 

 Embryotooy of the Chick, copyright 1951 by Blakiston Div., McGraw-Hill Book Co., printed 

 by permission. ) 



The mechanism of the cell movements that result in the shifting of the 

 presumptive areas of the blastula to their definitive positions in the gastrula 

 and neurula remains unknown. It is clear that the characteristic changes in 



157 



