48 HOW ANIMALS DEVELOP 



central nervous system, and also the skin. The 

 mesoderm develops into the main muscles and 

 skeleton of the body, and the endoderm forms 

 the gut and all the organs which are derived 

 from it later in development, such as the liver and 

 lungs, etc. 



Although gastrulation in the amphibian embryo is 

 more complicated than in the sea-urchin, it is obvious 

 that the two processes are similar in many ways. In 

 both the net result is to finish up with three layers, 

 and in both we find a blastopore and a primitive 

 gut. But in the amphibia the mesoderm is formed 

 directly from the skin of the blastula, and is not 

 given off secondarily by the endoderm. Moreover, 

 in the amphibian egg the process involves an active 

 movement down to the edge of the blastopore, then 

 a dive inside and movement again away from the 

 blastopore along the inner surface, while in the 

 sea-urchin the material simply folds in as a whole 

 and there is not so much movement over the edge 

 of the blastopore. 



Lampreys 



The '* stain experiment" has not yet been performed 

 on many groups of animals. Besides the amphibia 

 worked out by Vogt, Weissenberg has done experi- 

 ments with the lamprey, and Wetzel has given a full 

 account of the gastrulation of the chicken. We need 

 not give much description of the development of the 

 lamprey, since it is extremely like the process we 

 have just described in the newt. Fig. 11,^, shows 



