460 PRINCIPLES OF EMBRYOLOGY 



more caudal regions than does the overlying part of the neural plate. 

 This may perhaps find its explanation in Nieuwkoop's hypothesis of a 

 second phase of the induction process, in which the induced material 

 is gradually transformed into more posterior regions of the axis (p. 462). 



We find, therefore, that during the process of gastrulation, while the 

 mesoderm is exerting its inducing influence, it is itself only just in the 

 process of acquiring a pattern of regional specificities. It is these half- 

 formed specificities which must be responsible for the regionally different 

 types of induction which the various areas of the mesoderm exert. The 

 main question which has been debated recently is whether the specificities 

 within the mesoderm, and the various types of induction for which they 

 are responsible, are to be explained in terms of quantitative variations in the 

 concentration of some one substance, or whether the facts require us to 

 consider that different substances are being produced, characteristic of the 

 different regions. One may probably assume that by the time histological 

 differences can be recognised within the mesoderm sheet the various 

 regions have come to be characterised by particular substances. We have 

 seen (p. 216) that investigations on the inductive capacities of different 

 adult tissues have led to the conclusion that there are at least two different 

 inducing substances, one for forebrain and another for trunk-tail (or per- 

 haps solely for mesoderm). The question is, have such chemical differ- 

 ences arisen already at the time of gastrulation when the induction of the 

 neural plate first occurs, or is the individuation of that original induction 

 dependent only on a pattern of quantitative differences in the mesoderm ? 



It does not seem possible at present to give a perfectly firm answer to 

 this question. Most authors, however, seem to be agreed that the meso- 

 derm behaves as though composed of at least two different systems, one 

 corresponding to the prechordal plate and the other to the main mass of 

 chorda-mesoderm. It is to be presumed that these are chemically distinct, 

 but there is some divergence of opinion as to exactly what effects the two 

 regions produce, and as to the importance of quantitative variations within 

 them. Dalcq (1947) has recorded a series of investigations in which the 

 young gastrulae of Discoglossus were cut into two portions along some 

 parallel of latitude which separated the organisation centre into a more 

 animal (presumptively posterior) portion and a more vegetative pre- 

 sumptively anterior portion. The animal part is then rotated through 

 180 degrees and replaced on the lower in such a way that the posterior 

 organiser it contained lay on the ventral side (Fig. 20.25). Both parts 

 of the organiser invaginate and induce partial embryos. From the study 

 of such incomplete embryos, Dalcq came to the conclusion that the pre- 

 chordal plate always induces the forebrain or associated structures, and 



