The Development of the Vertebrate Embryo 



Most of the research carried out so far on gastrulation has been de- 

 scriptive, consisting of efforts to follow the patterns of the cell move- 

 ments, to trace the origins of the three basic tissue layers back into the 

 blastula and even to the fertilized egg, and to account for the tissues and 

 organs that are produced in the later embryonic stages. Now, biologists 

 can begin to ask questions at the mechanistic level. For example, what is 

 the motive force for gastrular movements? That is, are the cells pushed 

 from behind or pulled from in front, or does each cell move as an individ- 

 ual at the same rate as its neighbors? Why is it that the cells move to 

 specific areas of the embryo? What motivates somite cells to plaster them- 

 selves against ectoderm and endoderm respectively? By what mechanism 

 do endoderm cells move up and around so as to produce a tube-like gut? 

 Are these simply random movements that are channeled into specific pat- 

 terns by the shape of the embryo, or are the cells attracted or repelled by 

 specific chemical stimuli and then move toward or away from the places 

 where these substances are produced? 



As you can see, many problems of biological interest involving 

 gastrulation still remain unsolved and are open to investigation. 



Organ Formation 



Toward the end of gastrulation, ectodermal cells immediately in 

 front of the blastopore (or in front of the primitive streak of the chick 

 and mammal) begin to divide rapidly. The resultant crowding forces 

 some cells beneath the surface, and in this manner a thick plate of ecto- 

 derm appears. This is the neural plate. The wave of cell division proceeds 

 anteriorly and the neural plate finally extends along the entire dorsal line 

 of the embryo. Now the edges of the plate become elevated into folds 

 and the center of the plate is depressed. This produces a groove which 

 steadily deepens as the neural folds come together along the midline. 

 Finally the upper margins of the folds touch and fuse and the groove is 

 now formed into a tube, the neural tube, which is overlaid with a continu- 

 ous layer of ectoderm. Figure 31 shows these foldings in cross section, and 

 Fig. 32 shows the external appearance. As described below, the neural 

 tube is the rudiment of the central nervous system, including the brain 

 and the spinal cord. The embryo has reached the neurula stage. 



While the neural system takes shape, other parts change as well. 

 Externally, the embryo elongates and becomes sculptured into head and 

 trunk regions. Limb buds, tail buds, and gill slits appear. Internally, the 

 mesoderm splits into notochord and somites; the endoderm takes a long 

 trough-like shape and rolls up into a tube, the primitive gut, which is open 

 at the rear through the anus and later at the front through the mouth. 



