46 



HOW ANIMALS DEVELOP 



They have moved nearer together and lie underneath 

 the thick neural plate. The primitive gut is repre- 

 sented by a narrow hole, and a thick mass of 

 endoderm, which was made from the yolky cells 

 lying at the bottom of the blastula, inside the ring 

 of coloured patches. 



a b 



Fig. 1 1. — Vogt's map of the presumptive areas of the newt's gastrula 

 {a), and Weissenberg's map for the lamprey [b). Side view; the large 

 arrows ma^k the position of the blastopore. Widely spaced oblique 

 lines, skin; close oblique lines, neural plate; dotted, mesoderm; 

 white, endoderm. The directions of cell movements are also shown 



on Vogt's map. 



A large number of such experiments have been 

 made, and we know exactly where each part of the 

 blastula goes to during gastrulation. The easiest way 

 to summarize this information is to make a map, 

 showing for each region the organ into which it will 

 develop. It is usual to speak of mesoderm before it 

 has arrived at its final position as "presumptive 

 mesoderm" and of the material which will turn into 

 the neural plate as the "presumptive neural plate," 

 and so on. We can therefore call this map a map of 

 the presumptive areas. Vogt's map for the early newt 

 gastrula is given in a rather simplified form in Fig. 1 1 , a, 



