I. NEURULATION IQQ 



Hence, the cells of the vegetative field and the mai'ginal 

 zone, at the beginning of gastrulation, possess inherent capaci- 

 ties for self-differentiation, which even show local qualitative 

 differences. The animal field, on the other hand, gives an 

 entirely different picture. When isolated in a salt solution, 

 material from this area produces only irregular masses of cells, 

 in which the individual cells have an atypical structure. Only 

 if mesenchyme cells are present as well, vesicles will be formed, 

 the walls of which consist of an epithelium that is completely 

 similar to the epidermis of normal embryos. This shows that 

 here the capacity for self-differentiation is still very poor. On 

 the other hand, the range of possible differentiations is still 

 practically unlimited. A wide variety of transplantation ex- 

 periments has shown that the cells of the animal field can 

 differentiate either into muscle, or into nervous tissue, into 

 notochord, or kidney, gut or liver. In other words, practically 

 all the tissues of the body can be formed by this material. It 

 is still "omnipotent" (Holtfreter, 1933; Raven, 1935). There 

 are no local differences within the animal field in this respect. 

 Cells from the dorsal half, i.e. material of the prospective 

 neural plate, behave in entirely or practically the same way as 

 cells from the ventral half, i.e. prospective epidermis (cf. Fig. 

 35). If parts from these two regions of the animal field are 

 exchanged, they fit harmoniously into their new environments, 

 and can entirely substitute for each other (Spemann, 1918), 

 (Fig. 39). Evidently the cells of the animal field are of a 

 highly indifferent character; their chemodifferentiation is still 

 very slight. 



Now it is in this material that the progress of chemo- 

 differentiation during gastrulation, and the modification of 

 differentiation potencies involved, are manifested most clearly. 

 For a completely different situation is found if after gastrula- 

 tion, at the neurula stage, the potencies of the ectoderm are 

 again investigated by means of transplantations and explant- 

 ations. At this stage the ectoderm cells are no longer alike in 

 their differentiation potencies. A part of the neural plate will 

 produce nervous tissue, even if grafted into skin ectoderm 

 (Fig. 48), and, conversely, a piece of skin ectoderm, grafted 



