458 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



Vogt, Spemann, Mangold, and more recently by others. But quite apart 

 from this question, the lack of species-specificity suggests two possibilities 

 in this induction: production by different species and even by urodeles 

 and anura of the same, or closely similar, substances, with essentially the 

 same specilic effects; or an activating action dependent on the high physio- 

 logical level of the inductor and nonspecific. In other words, does the 

 inductor determine wholly or in part the specific character of the neural 







Fig. 158. — Transverse section through middle region of embryo shown in Fig. 157, D. 

 Primary embryo at left, secondary induced embryo at right of figure: fn, primarj^ neural tube; 

 in, induced neural tube; sc, secondary notochord; 5.y, secondary somite; sp, secondary pro- 

 nephric duct; sg, secondary gut. The implant, shown in lighter shading, forms the secondary 

 notochord and part of the right somite (after Spemann und H. Mangold, 1924). 



plate, or does it bring the ectoderm of a particular developmental stage to 

 a physiological level at which development of a neural plate takes place? 

 Consideration of further data must precede discussion of this question. 

 Both capacity of presumptive chorda-mesoderm to induce and of ecto- 

 derm to react change in the course of development. Exactly when induc- 

 tive capacity appears in the living embryonic inductor is diflicult to de- 

 termine because pieces from earlier pregastrula stages implanted in other 

 embr^^os continue to develop and may induce, and ectoderm is developing 

 at the same time. The reactivity of ectoderm gradually decreases in later 

 gastrula stages (Holtf refer, 1938a). 



