CHAPTER IX 



Induction and organisation 

 II. The period of organ development 



Neurulation is immediately followed by a rapid increase in the 

 spatial multiplicity of the embryo, caused by topogenetic pro- 

 cesses which now begin at various places in the body. During 

 gastrulation and neurulation, the topogenetic processes in the 

 germ formed a single coherent system, but now the processes 

 of folding, the cell migrations, and the invaginations and ex- 

 trusions, are of a more local nature. They result in the almost 

 simultaneous demarcation of a great number of organ primordia. 

 We shall now discuss this period of organ formation or 

 organogenesis. 



There is conclusive evidence that here, again, a major role 

 is played by the interactions of the parts of the germ which, 

 under the name of induction and organisation, we have studied 

 in the previous chapter. The development of the eyes in amphi- 

 bians may serve as a first example for the discussion of these 

 phenomena, because these organs were among the first to be 

 studied from the point of view of developmental mechanics, 

 and because analysis has already made very good progress here. 



The vertebrate eye is a complicated organ. It arises by the 

 combination of parts from different origins. After the neural 

 plate has closed, thereby forming the neural tube, both lateral 

 walls of the latter form a vesicular protrusion, the primary 

 eye-vesicle, in the region of the prospective fore-brain (Fig. 

 47 a). These protrusions at first have a wide communication 

 with the lumen of the neural tube, but this soon becomes 

 constricted so that the eye-vesicles are connected with the brain 

 only through the eye-stalks. The outer wall of the eye-vesicles 

 soon establishes contact with the skin ectoderm covering the 



