^ ECTODERM 



ENDODERM 



Fio. 19. — Grafting experiment in sea-urchins, {a) Shows the normal 



i6-cell stage; in {b) a top half of one egg (dotted) is combined with a 



side half of another, (c) Is a section of the resulting blastula showing 



some of the dotted material turned into endoderm 



