I. NEURULATION 



115 



Fig. 43. Diagram of the "insertion method"; (a) through a small 



incision, the implant is inserted into the blastocoel; (b) during 



gastrulation it is pushed against the ventral ectoderm of the host 



by the invaginating archenteron. 



employed in a great number of other investigations on the in- 

 ductive powers of certain implants. In Marx's experiments, 

 small but distinct neural plates developed in the ectoderm 

 overlying the grafts consisting of archenteron roof, proving that 

 this material possessed inductive powers. 



In this way the mechanism of the determination of the neural 

 primordium in normal development was elucidated. Position 

 and size of the neural plate evidently depend on position and 

 size of the underlying archenteron roof. Moreover, Spemann 

 (1931) demonstrated the existence of qualitative differences 

 within the organisation centre, which are responsible for the 

 regional differentiations of the neural plate. The first in- 

 vaginated part of the marginal zone, which becomes located at 

 the anterior end of the archenteron roof, induces especially the 

 cranial parts of the neural primordium, i.e. the brain, with eyes 

 and ear-vesicles. The material that invaginates later, and forms 

 the caudal parts of the archenteron roof, induces the caudal 

 regions of the neural primordium, i. e. the spinal cord (PI. VII) *. 

 It appeared that the influence of the archenteron roof extended 

 to the further development of the neural primordium as well. 

 Topogenesis and differentiation of the central nervous system 

 proceed in immediate dependence on the underlying notochord 



* Facing page 112. 



