CHAPTER X 



The later stages of development 



In the previous chapter we have seen how, in the course of 

 development, the embryo is subdivided into ever smaller groups 

 of cells, which are more or less independent, and have 

 different potencies. In the earlier phases of development, the 

 whole embryo behaves as one unit. It reacts to disturbances 

 with regulation, so that a more or less harmoniously built 

 embryo is formed in spite of the interference. In the later 

 stages, however, the situation is different. The power of regula- 

 tion, it is true, still exists within the individual organisation- 

 fields, but major disturbances, such as the removal or dis- 

 placement of complete organ primordia, can no longer be 

 regulated, and result in aberrant development. At this stage, 

 the embryo can be said to consist of a mosaic of building 

 stones which cannot be removed or displaced without damage 

 to its development. Each of these building-stones, the organ 

 primordia, develops more or less autonomously, and it can 

 no longer be replaced by any of the others. This phase may 

 therefore be called the ''mosaic stage^' of development. 



That, indeed, the organ primordia are irreplaceable at this 

 stage, is clearly proved by experiments in which the whole 

 tail-bud of young amphibian embryos was cut off. Such embryos 

 developed into tail-less larvae, nor was the missing part of the 

 body replaced at later stages (Schaxel, 1922), (Fig. 53). This 

 is the more remarkable because complete regulation would have 

 taken place if the material concerned had been removed at a 

 slightly earlier stage. Other material would then have formed 

 a tail. And if, on the other hand, the tail is amputated at later 

 stages, regeneration will occur, and a new tail will be formed 

 by a proliferation of cells (cf. Chapter XI). The mosaic stage, 



