APPENDIX 



491 



organised portions. When the embryos are almost but not quite 

 complete they are cyclopic (see pp. 245, 350). 



There is one further interesting point to be mentioned. When 

 only somitic mesoderm is invaginated, the resultant neural tube is 

 very small and is abnormal in cross-section; only in the presence 

 of notochord material will a full-sized and normally constructed 

 neural tube be induced (see p. 374). 



Fig. 221 



Regional inductive powers of exogastrulated organiser, a, diagram showing 

 experiment of placing pieces of undetermined epidermis on the anterior (i), 

 middle (2), and posterior (3) regions of the organiser (chorda-mesoderm) of 

 exo-embryo of axolotl. b, tail induced from epidermis in position 3. (From 

 Holtfreter, Biol. Zentralbl. liii, 1933.) 



A further set of important experiments was made by taking un- 

 determined ectoderm from normal early gastrulae and laying it on 

 different regions of the chorda-mesoderm of the exogastrulated 

 endo-mesoderm in the exogastrulation experiments. This must be 

 done in early stages, before the presumptive chorda-mesoderm has 

 disappeared under the surface of the endoderm (see fig. 221). 



The results are striking. From whatever presumptive region the 

 pieces of ectoderm were taken, they differentiate in accordance 

 with the regional properties of the organiser on which they are 

 lying. If placed on the anterior (head-organiser) part of the 



