MOVEMENTS AND FOLDINGS 47 



which also shows the directions in which the various 

 regions have got to move to get to their final 

 positions. The whole top of the blastula turns into 

 ectoderm, the part nearest the blastopore into neural 

 plate, the rest into skin. The presumptive endoderm 

 lies at the bottom, and the presumptive mesoderm 

 is an irregular ring between them, widest just in 

 front of the blastopore. It is surprising to see the 

 enormous movements which the tissues carry out. 

 The presumptive skin has to expand so as to cover 

 the whole surface ; the presumptive neural plate has 

 to swing in towards the middle line, while the 

 presumptive mesoderm gets out of its way by diving 

 into the blastopore and growing forwards along the 

 inside. The endoderm is more passive, and is carried 

 along by the other parts of the embryo. Nobody 

 knows quite what makes these movements happen, 

 or where the force comes from to pus-h the tissues 

 along. But already at the beginning of gastrulation 

 the various regions have tendencies to behave in the 

 appropriate way. If different bits of tissue are cut 

 out and grafted back into other parts of the blastula 

 (by a method which will be described more fully 

 later), they go on behaving in their own typical 

 fashion even in the wrong situation ; the presumptive 

 skin stretches in area, the presumptive mesoderm 

 sinks in and disappears, and so on. 



If we leave the coloured patches a bit longer, we 

 can soon find out what the three layers develop into. 

 We find that, as was said above, the ectoderm forms 

 the neural plate, which turns into the brain and 



