EMBRYONIC INDUCTORS AND ORGANIZERS 



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circular, gradually decreases in diameter, and overgrows the yolk. The 

 invaginated cells migrate anteriorly beneath the ectoderm and give rise 

 to notochord and mesoderm, and continued increase in area of the super- 

 ficial cells brings more material to the blastopore lip and so to invagination. 

 Isolation and transplantation experiments have brought to light various 

 facts of interest concerning these regional changes in position of cellular 

 material. Vogt (1923) maintains that they are not the summation of ac- 

 tivities of single cells but essentially regional amoeboid movements in 



A B 



Fig. 151, A, B.— Directions of cell migrations in amphibian gastrulation. A, lateral, B, 

 dorsal view; alternation of broken and unbroken lines merely for ease in following directions 

 (from Vogt, 1929). 



which the individual cells are passive. Pieces of the future blastopore lip 

 transplanted to other regions undergo the same "stretching" and invagi- 

 nation as in normal environment, even when abnormally oriented with 

 respect to the region in which they are implanted, and different regions 

 of the blastopore lip stretch and invaginate independently. When invagi- 

 nation is prevented in transplanted pieces or united halves of embryos, 

 hornlike or irregular outgrowths arise. Also, stretching and invagination 

 may occur independently of each other. According to Holtfreter, how- 

 ever, the abihty to stretch is intrinsic in individual cells; isolated blastula 

 cells undergo the change of shape in the same way as larger cell groups.'^ 



Thus far no physiological interpretation of these changes in shape and 

 migrations of cells has been offered, but it is perhaps of interest to note 



6 0. Mangold, 1920; Vogt, 1922&; Spemann und H. Mangold, 1924; Spemann, 1931, 1936, 

 pp. 63-71, 1938, pp. loi-io; Holtfreter, 1939'^- 



