108 



GASTRULATION 



Invagination — As used by Vogt, this term means insinking (Ein- 

 stulping in German) of the egg surface followed by the forward 

 migration (Vordringen) which involves the displacement of inner 

 materials. Schechtman uses the term to mean an inward movement, 

 without any reference to whether there is pulling, pushing, or autono- 

 mous movement. There is probably some invagination in gastrulation 

 of the frog's egg. 



Involution — As used by Vogt, this term means a turning inward, 

 a rotation of material upon itself so that the movement is directed 

 toward the interior of the egg. Involution does occur in the frog's egg 

 gastrulation. 



Delamination — This is a splitting-oflf process whereby the outer 

 layer of ectoderm gives rise to an inner sheet of cells known as the 

 endoderm. It does not occur in the frog's egg gastrulation. 



Epiboly — This is a progressive extension of the cortical layer of 

 the animal hemisphere toward the vegetal hemisphere, or a sort of ex- 

 pansion from animal toward the vegetal hemisphere. In some eggs 

 (e.g., the mollusk Crepidula) it actually involves an overgrowth by 

 smaller cells over the larger vegetal hemisphere cells. The extension 

 type of epiboly does occur in the frog's egg gastrulation. 



Extension — This term is used by Schechtman to describe in am- 

 phibia the self-stretching process which seems intrinsic within certain 

 cell areas, particularly in the dorsal region of the marginal zone. It is 



Diagrams to show the directions of movement and the displacement of the 

 parts of the blastula in the process of gastrulation in amphibia. (After W. Vogt, 

 1929b. From Spemann: "Embryonic Development and Induction," New Haven, 

 Yale University Press.) 



