I. NEURULATION 



ii: 



Uo* 



secondary embryo forms a distinct head with brain, eyes, ear- 

 vesicles, etc., while its posterior end develops into a tail bud. 

 The secondary embryo is then distinguishable from the primary 

 embryo only by its slightly smaller size (PI. VI) *. For these 

 reasons, Spemann called such a graft an ''organiser", and the 

 dorsal marginal zone from which it originated the ''organisa- 

 tio7i centre'' of the embryo. In this way he voiced his conviction 

 that this area played a role in 

 normal development as well. Bautz- 

 mann (1926) determined the exact 

 limits of the organisation centre. 

 At the beginning of gastrulation, 

 the organising power, or, more 

 accurately, the power to induce a 

 neural plate, was found to be limit- 

 ed to the dorsal half of the margin- 

 al zone, i.e. the area which in 

 normal development provides the 

 notochord and somites (Fig. 41). 

 The first experiments of Spe- 

 mann and Hilde Mangold had al- 

 ready shown that in secondary 

 embryos the organ boundaries did 

 not necessarily coincide with those 

 between graft and host. In some 



cases, part of the graft was not rolled in, but remained at the 

 surface, and associated with the ectoderm. This was then 

 incorporated into the neural plate of the secondary embryo, 

 so that this organ was a "chimaera", consisting partly of graft 

 tissue, and partly of host tissue (Fig. 40 b, d). A similar 

 chimaerical composition was sometimes encountered in the 

 notochord and somites of the secondary embryo, where cells of 

 the host's ventral mesoderm, bordering on the graft, took part 

 in the formation of these organs (Fig. 40 d). This means that 

 the inductive activity of the graft did not only manifest itself 

 in the overlying ectoderm, and the underlying endoderm; it 

 had evidently also extended from cell to cell within the meso- 

 derm, and forced a development in an aberrant direction on 



* Facing page 97. 



Rav^en - Outline Physiologie 8 



Fig. 41. Extent of the orga- 

 nisation centre (dotted) in 

 the initial stage of gastru- 

 lation of Triton taeniatus. 

 Germ seen from the left; 

 the wavy line indicates the 

 blastopore furrow. After 

 Bautzmann. 



