34 



EMBRYOLOGY 



organization of the egg (Fig. 13), vertical cuts might be predicted to form 

 a whole embryo because when vertical cuts are made the resultant pieces 

 include a sample of the mesodermal, the endodermal, and the ectodermal 

 substances. However, it should be pointed out that the cut pieces contain 

 only a half part of each of these three regions, and one could hardly expect a 

 bilaterally symmetrical embryo from half an egg. If the parts of the embryo 



DAUERBLASTULA 



DAUERBLASTULA 



Fig. 15. Chemical control of the formation of endoderm and ectoderm. 

 Left: Sea urchin egg cut in two through the equator. Animal half fertilized 

 develops into dauerblastula without endoderm. Animal half treated with lithium 

 forms embryo with endoderm. Right: Whole egg treated with NaSCN before 

 fertilization. After fertilization the egg develops into a dauerblastula without 

 endoderm. What has happened to the egg substances which normally form 

 endoderm ? 



