46o PATTERNS AND PROBLEMS OF DEVELOPMENT 



THE LONGITUDINAL DIFFERENTIAL IN INDUCTOR 

 AND IN REACTING ECTODERM 



The region of the dorsal inductor lying nearest the level at which blasto- 

 pore formation begins, and invaginating first, migrates anteriorly after 

 invagination and finally comes to underlie the future head region, while 

 parts which invaginate later underlie successively more posterior regions. 

 In the attempt to determine whether a longitudinal differential or specific- 

 ity in inducing capacity exists, pieces of the dorsal lip at different stages 

 of gastrulation have been implanted. Pieces from the earliest stages of 

 gastrulation implanted at the level of the future head induce neural plates 

 with well-developed head region, but when implanted at trunk-levels also 

 induce well-developed heads. Pieces from the dorsal lip of later stages 

 implanted at head-levels may also induce neural plates with anterior 

 head region approaching normal; but when implanted at more posterior 

 levels, the anterior region of the induced neural plate is deficient or absent. 

 Pieces of the dorsal inductor invaginating last and lying farthest posteriorly 

 after invagination may induce tail.'-' These results have led to the desig- 

 nation of different levels of the inductor as "head-inductor," "trunk- 

 inductor," and "tail-inductor." Essentially similar results have been ob- 

 tained with implants of different regions of the chorda primordium alone 

 (Bautzmann, 1928, 1929&). However, as Spemann has pointed out, the 

 longitudinal differences in results depend not only on regional difference 

 in the inductor but also on a difference in the reacting ectoderm, for 

 "trunk-inductor" implanted at head-level induces head more or less com- 

 pletely and is completely trunk-inductor only at more posterior ectoder- 

 mal levels. 



Before invagination the physiological gradient in the inductor decreases 

 anteriorly from the region invaginating first, and in the ectoderm of the 

 presumptive neural plate and in other ectoderm there is a gradient differ- 

 ential decreasing from the region about the apical pole (pp. 151-58). 

 After invagination the two gradients coincide in direction and more or 

 less closely in extent, in the region developing as neural plate, the one 

 underlying the other. When the high gradient-level of the inductor is 

 implanted at the high ectodermal level, a well-developed head results; 

 but the high inductor level is also able to induce more or less complete 

 head development at lower levels of the ectodermal gradient, and a lower 

 inductor level may induce head when implanted at the high ectodermal 



'3 Spemann, 1931, 1936, pp. 167-74, 1938, chap, xiii; Holtfreter, 1933^, 1936; Bytinski- 

 Salz, 1931; O. Mangold, 1932^; Lehmann, 1932; Schechtman, 1938. 



