MOVEMENTS AND FOLDINGS 53 



movement begins within the streak. The streak, in 

 fact, gets shorter again, telescoping up along its 

 length. The movement causes the front part to be 

 drawn out, like glass in a flame, and as it becomes 

 thinner and longer this part separates off from the 

 rest of the streak and becomes the neural plate and 

 the mesodermal rudiment of the backbone. This 

 mesoderm does not remain as a flat plate, but soon 

 breaks up on each side of the neural plate into a 

 series of little lumps, the body-segments, which are 

 found in all embryos at this stage, but are more 

 obvious in the chick than in the other embryos 

 which have been described. Between the two rows 

 of segments, immediately beneath the neural plate, 

 is a continuous strand of mesoderm, which is the 

 first part of the backbone to be developed. 



In a map of the presumptive areas at the stage 

 w^hen the primitive streak is fully grown but before 

 the backward movement has begun, the presumptive 

 neural plate and presumptive body segments and 

 backbone are concentrated at the front end. The 

 map in Fig. 14 shows this clearly for the neural 

 plate. It is not very easy to compare this with the 

 map for the newt, because in the newt the endoderm 

 and mesoderm are formed together and in the chick 

 are formed separately. The chicken has what may 

 be called an "endoderm-blastopore" in the posterior 

 of the blastoderm before the primitive streak appears, 

 and Graper thinks this should be compared with 

 the newt blastopore; but Wetzel thinks the com- 

 parable structure is the later "mesoderm-blastopore" 



