ORGAN DEVELOPMENT IN VERTEBRATES 



255 



tapetum develops into part of the pigmented and protective coverings of 

 the eye. 



The second ingredient in the developing eye is the lens. This originates 

 from an infolding of ectoderm at the point where the optic swelling 

 touches it. Its inception normally depends on a process of induction. 

 Some time before Spemann discovered that the appearance of the main 

 axis of the embryo depends on induction by the primary organisation 

 centre, he had found that the eye-cups of newt embryos, transplanted so as 

 to come in contact with the ectoderm of the belly, caused the appearance 

 there of an induced lens (Reviews: Mangold 193 1, Spemann 1938, Need- 

 ham 1942). 



The experiment reveals a classical example of a secondary organiser, 

 that is, one which is effective after the primary organiser activity is com- 

 pleted. Very many of these operate during the period in which the earliest 

 organ rudiments are appearing (Fig. 12.3). In fact it is probable that in the 



Ear capsule 



I Forebrain] | Midbrain] \ |Hindbrain| 



"Lumen of Kut Neural tube T 



Figure 12.3 



Diagram of secondary organisers in the newt. 

 (After Hollfreter.) 



vertebrates every rudiment owes its origin to a secondary organiser or to 

 a complex of organising influences ; they are found in all the vertebrate 

 classes which have been studied. For instance, lens induction by the eye- 

 cup has been demonstrated by Waddington, van Deth and others in the 

 chick (Review: Waddington 1952^7), And to give further examples, two 

 other organs which require notice at this point also depend on secondary 

 organiser complexes. The nervous component of the nasal organ arises 

 as two ectodermal thickenings or placodes near the anterior end of the 

 forebrain; and it can be shown by transplantation experiments that the 

 forebrain induces them. Again, the ears originate near the hindbrain from 

 similar lens-like ectodermal thickenings, around which, after they have 

 been folded inwards, the mesodermal structures are later formed. Experi- 

 ments have shown that in this case the inducing system is complex; the 



