MOVEMENTS AND FOLDINGS 43 



after embedding them in wax, they can be cut into 

 thin slices or sections. These sections are then stained 

 to bring out the different structures, and one can 

 thus find out what the internal anatomy of the 

 embryo is like. But for investigating the changes 

 going on in gastrulation a very much improved 

 method has been recently worked out, chiefly by 

 Professor Vogt of Zurich. When gastrulation is 

 beginning, the embryo is placed for a few minutes 

 against Httle blobs of jelly which have been soaked 

 in dyes. The cells of the embryo which are in contact 

 with the jelly absorb the dye and become stained 

 themselves, showing up as coloured patches on the 

 surface of the blastula. If the dye is chosen rightly 

 it is not poisonous, and the cells remain quite 

 healthy and the embryo proceeds with its gastrula- 

 tion. All one has to do in order to follow the 

 gastrulation is to watch the dyed patches and see 

 how they move. The whole process can be followed 

 in one and the same embryo, and there is no need 

 to rely on the comparative study of a whole set of 

 specimens of different ages. 



Fig. 9, p. 49, shows photos of a newt embryo during 

 gastrulation. In the first photo gastrulation is just 

 beginning, and a slight crescent-shaped groove has 

 appeared on the surface of the blastula. This is the 

 beginning of the blastopore. In the newt it does not 

 He right at the bottom, as it did in the sea-urchin, 

 but rather to one side (see Fig. lo, a). From the 

 outside, without the aid of coloured marks, all we 

 can see is that the blastopore first becomes bigger, 



