ELEMENTARY MORPHOGENESIS 



43 



of the animal, at least in certain of its parts not in all of 

 them is changed again from bilateral to radial, just the 

 opposite of what happened in the very early stages ! 



But we cannot follow the embryology of our Echinus 

 further here ; and indeed we are the less obliged to do so, 

 since in all our experimental work we shall have to deal 

 with it only as far as to the pluteus larva. It is impossible 



Fio. 4. LARVAL DEVELOPMENT OF ECHINUS. 



A. The gastrula. 



B. Later stage, bilateral-symmetrical. Intestine begins to divide into three parts. 



C. Pluteus larva. S = Skeleton. I = Intestine. 



under ordinary conditions to rear the germs up to the adult 

 stages in captivity. 



You now, I hope, will have a general idea at least of the 

 processes of which the individual development of an animal 

 consists. Of course the specific features leading from the 

 egg to the adult are different in each specific case, and, in 



