EMBRYONIC INDUCTORS AND ORGANIZERS 463 



Explanted presumptive entoderm develops as entoderm and may attain 

 a high degree of differentiation. 



Various eariier attempts to culture isolated parts of amphibian blastu- 

 lae and gastrulae in organic media proved unsuccessful, but it was dis- 

 covered that pieces introduced into the coelom of an older larva would live 

 and develop. Development of these coelomic cultures differs in certain 

 respects from that in salt solutions, as regards presumptive epidermis and 

 neural plate (Holtfreter, 1929, 1931a). Presumptive neural plate may de- 

 velop either into pure nervous tissue or into epidermis with practically equal 

 frequency; and presumptive epidermis, into epidermis with differentiation 

 of gland, ciliated, and pigment cells or into nervous tissue. Implantation 

 in the eye cavity of older individuals after removal of the eye again gives 

 different results; chorda-mesoderm may develop from almost all regions 

 of the early gastrula of Triton (Kusche, 1929), even from the region of the 

 apical pole in Amhly stoma (Bautzmann, 1929a). 



Developing eggs of the urodele Tritums, subjected to a lateral tempera- 

 ture gradient, may develop on the warm side thickenings resembling neu- 

 ral tissue in structure from regions of presumptive epidermis not under- 

 lain by chorda-mesoderm and typical neural folds from regions with un- 

 derlying mesoderm (Gilchrist, 1928, 1929a, 1933). Other experiments with 

 localized high temperatures have not produced clearly identifiable neural 

 tissue (Castelnuovo, 1932; Margen and Schechtman, 1938). Inhibition of 

 one-half of the embryo by low temperature or by lack of oxygen results in 

 asymmetry and alteration in developmental fate of various parts. It 

 shows, further, that different regions of the neural plate may develop in- 

 pendently of each other and that the anterior head region may develop 

 when gastrulation is almost completely suppressed (Vogt, 1927; 1928a, b). 



Naked embryos (axolotl) developing from early blastula stage in a 

 modified Ringer solution undergo exogastrulation, the mesentoderm evag- 

 inating instead of invaginating and in the complete exogastrula never 

 underlying any part of the ectoderm (Holtfreter, 1933^, e). Directions of 

 cell migrations in exogastrulation are indicated in Figure 159, A, B; the 

 differentiation, in C. Ectoderm of these exogastrulae forms an irregular 

 cell mass, not a definite layer, and shows no traces of neural plate or 

 neural tissue and no evidence of any definite developmental pattern. It 

 may become ciliated, but the ciliary beat is not definite in direction, as in 

 the normal animal. The evaginated mesentoderm, on the other hand, 

 undergoes a high degree of differentiation into notochord, muscles, kidney 

 tubules, and gut and may approach the normal embryo in general form. 



