176 PRINCIPLES OF EMBRYOLOGY 



and sectioned material and this made it possible to demonstrate con- 

 clusively that the grafted tissues had not only themselves developed into 

 parts of axial organs, but had also caused the surrounding host tissue 

 to do so, although its presumptive fate was to become mere epidermis. 



ind.n.p. 



hnf 



Figure 10.2 



1. View of vegetative pole of a newt gastrula. The main blastopore is to- 

 wards the bottom, and a second blastopore region, from another gastrula 

 which was vitally stained with Neutral Red, has been grafted into the ven- 

 tral side of the vegetative region. 



2, 3. Stages in the invagination of the normal and grafted blastopores. 



4. The grafted material has become completely invaginated and disappeared 

 below the surface. It has induced a neural plate (/»</. n.p.). One of the normal 

 neural folds of the host embryo can be seen (h.n.p.). 



(Original, from a time-lapse cine film.) 



Such a reaction was spoken of as an embryonic induction, and the region 

 near the blastopore from which inducing grafts can be obtained was 

 called by Spemann the organisation centre or organiser of the embryo. 



The discovery of the organiser gave embryologists for the first time 

 the power to control the direction in which an embryonic tissue develops, 



