INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 46I 



that its effects can be arranged in a series of grades corresponding to the 

 amount of inducing material present. The lowest grade of induction, he 

 claims, is an isolated pineal body. When larger quantities of prechordal 

 plate are present a small forebrain vesicle is induced. The next stage in- 

 duces the appearance of a fmger-like anterior expansion which is capable 

 of inducing an olfactory placode. With still more intense inductive action 

 the eyes appear. 



Dalcq suggests that similar variations in effectiveness, dependent on 

 quantitative differences in the inducer, occur also in more posterior parts 



Figure 20.25 

 Figure a, a young gastrula of the anuran Discoglossus is marked with a vital 

 stain above the blastopore, and then cut in two along a parallel of latitude; 

 b, the upper half is rotated through 180 degrees and replaced; c, a normal 

 embryo is usually formed on the original dorsal side (unless the cut was too 

 low), and a partial embryo, lacking some part of the anterior region, appears 

 on the ventral side. (After Dalcq 1947.) 



of the nervous system overlying the chordamesoderm. The most clear-cut 

 evidence in regard to these regions concerns the medio-lateral extent of 

 the plate rather than its anterior-posterior structure. There is considerable 

 evidence that the inductive capacity of the chorda-mesoderm is most 

 powerful in the midhne and decreases on either side. Thus it can be 

 shown that at the late gastrula stage the lateral plate mesoderm has a weak 

 capacity of induction which can be effective on the highly reactive ecto- 

 derm of the young gastrula stage, but not on the less-reactive ectoderm of 

 older gastrulae (Waddington 1936^). Raven and Kloos (1945) have made 

 grafts from medial or lateral parts of the archenteron roof underlying the 

 neural plate and showed that the lateral ones were weaker inducers. Again, 

 two of Dalcq's students, Damas (1947) and Gallera (1947), have studied 

 the effects of cutting short the period in which induction can proceed. At 



