44 HOW ANIMALS DEVELOP 



and the horns of the crescent spread round till they 

 join up to form a circle, which encloses the whole 

 yolky bottom part of the egg; it then becomes 

 smaller again, pushing the yolky cells inside, and 

 closes up to a tiny slit, and eventually disappears. 

 Just about at the slit stage a sinuous ridge appears 

 on the surface of the embryo, marking off a horseshoe- 

 shaped area, which is darker in colour than the rest. 

 The sides rapidly get nearer together, and the 

 horseshoe-shaped area becomes squeezed up to a 

 dumb-bell shape, and finally to a deep groove. The 

 dark area is known as the neural plate, or groove, 

 according to the stage at which it has arrived. It is 

 the first rudiment of the brain and central nervous 

 system, and, as we shall find, gastrulation is com- 

 pleted before it appears; an embryo in which the 

 neural plate can be seen is no longer called a gastrula 

 but a neurula. 



Fig. 10 shows a stain experiment from which one 

 can discover what is really going on all this time. 

 The first figure shows a series of patches made at 

 the beginning of gastrulation, arranged in a ring 

 round the blastopore. In the stages illustrated in the 

 next two figures the blastopore becomes round and 

 then closes up to a slit, and as it does so the patches 

 move in towards it and disappear inside, so that 

 when the neural plate has developed, in Fig. lo, ^, 

 nearly all the coloured patches have been swallowed 

 up. A section through the embryo at this stage shows 

 that all three layers have been formed and that the 

 coloured patches lie in the middle layer or mesoderm. 



