50 HOW ANIMALS DEVELOP 



cap of cells lying on top of the yolk but separated 

 from it by a small cleft representing the blastula- 

 cavity. 



As in the sea-urchins, the gastrulation takes place 

 in two stages, first the separation into ectoderm and 

 endoderm and then the formation of the mesoderm. 

 In birds the mesoderm is developed from the ecto- 

 derm, not from the endoderm. The endoderm for- 

 mation takes place very early, usually before the egg 

 is laid. In the posterior part of the little disc, which 

 is called the blastoderm, cells sink down and then 

 turn underneath and grow forward along the under 

 surface until the whole area is provided with a 

 second layer, with ectoderm above and endoderm 

 below. When the egg of a chicken is laid, the blasto- 

 derm is already in this two-layered condition, and 

 it is also divided into a central transparent area, 

 where the embryo will develop, and an outer ring 

 which is opaque because the endoderm cells there 

 contain a great deal of yolk. The outer opaque area 

 develops into various specialized organs for absorbing 

 the yolk, and will not concern us any more in this 

 chapter. 



The changes which go on in the transparent area 

 have been investigated by two new methods, and 

 although their results are in almost complete 

 agreement with one another, it is difficult to recon- 

 cile some of their details with observations based 

 on the old methods. One of the new methods is 

 similar to Vogt's, and depends on watching what 

 happens to coloured patches made with non-poison- 



