466 PATTERNS AND PROBLEMS OF DEVELOPMENT 



body when it is isolated from higher levels of the gradient pattern and 

 sufficiently activated. In the temperature experiments the whole gra- 

 dient pattern of the parts subjected to the higher temperature is elevated 

 to a higher level, but presumably there is little or no change in the rela- 

 tions of parts and consequently little or no alteration in the course of 

 development. 



The conclusion of Goerttler and Lehmann that the anterior region of 

 the presumptive neural plate is more capable of independent differentia- 

 tion than more posterior levels is in accord with the evidence concerning 

 gradient pattern. This region is the region about the apical pole, the high 

 end of the primary gradient. Under certain conditions its gradient-level 

 is apparently high enough, relative to other parts, to permit independent 

 development as neural plate or neural tissue, while at more posterior 

 levels there must be activation, relative to surrounding ectoderm, either 

 by the inductor or, in isolated pieces, by environmental conditions. 



Vogt and Spemann agree in admitting a Bahnung toward determina- 

 tion in the presumptive neural plate, independent of the invaginated in- 

 ductor. But that the determination of presumptive epidermis or neural 

 plate has not become highly specific and fixed, even in the gastrula, is 

 indicated by other results of transplantation. Presumptive ectoderm at 

 the beginning of gastrulation, implanted in presumptive mesodermal or 

 entodermal regions, becomes mesoderm or entoderm (O. Mangold, 1923). 

 Transplanted to dorsilateral regions of older embryos, both presumptive 

 epidermis and neural plate may develop, the neural part ranging from 

 brain with nose and eye to posterior parts of the spinal cord, according to 

 level of implantation. It may also take part in balancer formation or in de- 

 velopment of gill or leg, and its deeper layers may become mesoderm. In 

 the pronephric region it may give rise to pronephric tubules; in contact 

 with muscle it may form muscle, this most frequently at more posterior 

 levels, or notochord, or even gut wall (Holtfreter, 19336). 



The relative significance of gradient-levels as such and of specific re- 

 gions or fields. in bringing about these inductions is not known. The ques- 

 tion whether tissue of presumptive neural plate can become neural tissue 

 independently of the inductor and of other parts of the embryo appears 

 to have been finally settled by the experiments of Barth (1939c). Explants 

 of presumptive neural plate with presumptive epidermis from stages of 

 Ambly stoma punctatum in which these tissues still form the roof of the 

 blastocoel and before the inductor underlies the presumptive neural re- 

 gion, when fused by their anterior ends, form neural tubes in the region 



