Amphibians 



and their inherent polarity was apparently 

 changed by the surrounding tissue.) How- 

 ever, the mediolateral axis was found to be 

 still reversible in the medullary plate stage 

 (Roach, '45). The same holds for the hind- 

 brain field; the extirpation of its lateral half 

 is followed by restitution from the intact half 

 (Harrison, '47). 



Systematic studies of Mangold ('31a, '36) 

 and Adelmann ('37) lead to the conckision 

 that the normal segregation of the field into 

 brain and bilaterally arranged eyes is con- 



253 



Instead of using surgical methods, one can 

 prevent the development of the prechordal 

 plate by exposing early gastrulae to certain 

 concentrations of lithium chloride. In this 

 case, a continuous layer of mandibular head 

 mesoderm forms across the midline, and the 

 induced structures exhibit again the syn- 

 drome of a Cyclopean head (Lehmann, '33; 

 Adelmann, '34). These I'esults indicate that 

 the bilateral disposition of the material for 

 forebrain and eyes is decisively influenced 

 by the corresponding mass distribution of the 



Fig. 88. Semidiagrammatic illustration of the formative effects of somites, notochord and mesenchyme upon 

 the shaping of the neural tube; see text. (Original.) 



trolled by formative effects of the underly- 

 ing parts of the mesoderm. Following ex- 

 tirpation of the whole prechordal mesoderm 

 from a neurula, 75 to 100 per cent of the 

 embryos developed all degrees of synophthal- 

 mia to Cyclopean abnormalities. The percent- 

 age of defects was approximately the same 

 when only the median "prechordal plate" 

 (Fig. 85) was extirpated, but it dropped to 

 44 per cent when only the paraxial "man- 

 dibular" portions of the mesoderm had been 

 removed. Conversely, grafts containing the 

 median strip of the anterior neiiral plate 

 without the subjacent prechordal plate pro- 

 duced only a single eye, but when grafted 

 together with this substratum, they formed 

 two complete eyes connected by brain tissue, 

 in 70 per cent of the cases. 



subjacent mesoderm, especially the pre- 

 chordal plate, and that this influence is 

 exerted even after the brain-eye field has 

 been induced. 



It is doubtful whether this continued for- 

 mative effect of the mesodermal substratum 

 should be classified as an "induction." Adult 

 tissues having no axial organization can like- 

 wise induce brains with bilaterally arranged 

 sense organs (Holtfreter, '34b; Chuang, '39; 

 Toivonen, '40). This makes it likely that the 

 prechordal mesoderm merely provides the 

 unspecific mechanical conditions for the 

 bilateral segregation of the induced field. 

 As such this influence would belong in the 

 same category as that of the (non-inductive) 

 mesenchyme which is necessary as a sup- 

 porting matrix for the transformation of the 



