INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 465 



when energetic morphogenetic movements are going on; the mesoderm 

 is invaginating at the blastopore and is moving forward under the pre- 

 sumptive neural plate, which at the same time is streaming backwards 

 above it towards the blastopore. Nothing very defmite is known about 

 the significance of these movements for the processes of regionalisation, 

 but it seems almost impossible to beheve that they are in fact without 

 importance. It seems probable, indeed, that it is to the movements of 

 gastrulation that one should look for an explanation of the field of quanti- 

 tative variation in inducing capacity which all authors seem to agree must 

 exist witliin the mesoderm. If an evocating substance begins to be re- 

 leased in the mesoderm at the time this invaginates through the blastopore, 



Figure 20.27 



Diagram to illustrate the results of partial inhibition of the development of 

 the brain. The upper row shows, on the left, the normal situation with two 

 fields, for forebrain and for mid- and hindbrain. On the right of this row are 

 three stages of inhibition ; note that the more posterior field is not reduced 

 at all until the anterior one has disappeared entirely. Below are diagrams of 

 the types of brain corresponding to the fields, with indications of the tele- 

 cephalon, diencephalon, mesencephalon and myelencephalon. (From Leh- 



mann 1948.) 



