72 HOW ANIMALS DEVELOP 



Mammals 



Organizers have been found in other groups still 

 more widely removed from newts. We know rather 

 little about the organizers in mammals : in man we 

 have no certain knowledge at all of them. But it has 

 been proved that the rabbit embryo can react to a 

 chick primitive streak organizer and can be induced 

 by it to develop an extra neural plate. This suggests 

 that the mechanism is essentially the same in 

 mammals as in the other groups and that they also 

 have organizers, but it has not yet been possible to 

 make organizer grafts because of the technical 

 difficulties mentioned before (p. 64). This experi- 

 ment also shows how extremely unspecific organizers 

 are. It seems that an organizer from any group 

 of animals will work on the embryos of any other 

 group. Probably, as is discussed later (p. 94), the 

 activity of the organizer is due to a chemical sub- 

 stance, and this may very likely be the same substance 

 in all the different groups of vertebrates. 



Sea-urchins 



Among the invertebrates, Horstadius has discovered 

 an organization centre in the sea-urchin embryo 

 which is very like, though slightly different from, 

 that of the higher animals. The sea-urchin's tgg^ as 

 was described earlier, has a very simple gastrulation ; 

 the blastula is spherical, with all the cells more or 

 less equal in size, and in gastrulation the bottom of 

 it just draws back into the inside. Gastrulation 



