332 



GENERAL PRINCIPLES 



FIG. 200. 



FIG. 201. 



FIG. 202. 



FIG. 203. 



FIGS. 200 to 203 represent four steps in the development of the larva from the 

 gastrula. The gastrula mouth narrows and is finally closed in by the ectoderm. 

 The gastrula cavity elongates and becomes the primitive digestive cavity 

 (archenteron). 



