organisers: inducers of differentiation 135 



layer after gastriilation, at which time the organiser has been in- 

 vaginated, and forms the primitive gut-roof, i.e. notochord and 

 axial mesoderm (future myotomes). In both cases, the hinder part 

 of the presumptive neural fold region will be affected before the 

 front part. 



It appears that the organiser acts in both these ways. That it can 

 exert its inducing action from below, after gastrulation, is demon- 

 strated by the fact that when a graft is made from the dorsal lip of 

 the blastopore of one gastrula into the flank of another blastula or 

 gastrula, it brings about the formation of the essential structures 

 (so-called axial structures) of an embryo. This embryo is called the 

 secondary embryo in order to distinguish it from the primary 

 embryo formed from the tissues of the host in the ordinary way. ^ 



The secondary embryo arises from tissue which had very differ- 

 ent prospective fates. The grafted organiser invaginates beneath the 

 surface of the tissues of the host and itself gives rise to part or all of 

 the notochord and axial mesoderm of the secondary embryo. The 

 other structures of the secondary embryo are usually formed of 

 host tissue only, but may contain an admixture of graft tissue also. 

 These can be easily distinguished by performing the experiment 

 with material derived from two species of Triton, T. cristatus and 

 T. taeniatiis, which differ in the pigmentation of their tissues. 



There is therefore no doubt that the organiser can bring about 

 the determination of tissues by the transmission of stimuli from 

 underneath after gastrulation. This is further proved by the fact 

 that pieces of the primitive gut-roof (notochord and mesoderm, 

 which of course are derived from invaginated organiser material) 

 are capable of inducing the formation of axial structures.- \s we 

 shall see later, the main activity of the organiser in normal develop- 

 ment is to induce the formation of the neural plate and tube. This 



^ In all cases, portions of central nervous system, notochord, and axial meso- 

 derm (somites) are formed ; in addition, brain and spinal cord, eyes, ears, kidneys, 

 peripheral mesoderm (lateral plate), gut-roof and heart may be produced. Such 

 embryos have not been kept beyond the tail-bud stage. Whether certain organs 

 of the secondary embryo are formed or not<iepends on several factors: (i) the 

 level of the host's main axis at which the graft is made ; (2) the region of the 

 organiser which is used as a graft; (3) the distance of the primary from the 

 secondary embryo, resulting in a greater or lesser degree of mechanical inter- 

 ference. (Spemann and Mangold, H., 1924.) 



^ Marx, 1925; Bautzmann, 1926. 



