ELEMENTARY MORPHOGENESIS 2*7 



ontogenesis in the descriptive sense ; the knowledge of the 

 fact of this process must be the very foundation of all 

 studies on the theory of development in any case, and 

 therefore we shall devote this whole lecture to studies in 

 merely descriptive embryology. 



But descriptive embryology, even if it is to serve merely 

 as an instance of the universality of the fact of epigenesis, 

 can only be studied successfully with reference to a concrete 

 case. We select the development of the common sea-urchin 

 (Echinus microtiiberculatus) as such a case, and we are 

 the more entitled to select this organism rather than another, 

 because most of the analytical experimental work, carried 

 out in the interests of a real theory of development, has 

 been done on the germs of this animal. Therefore, to know 

 at least the outlines of the individual embryology of the 

 Echinus may indeed be called the conditio sine qua, non 

 for a real understanding of what is to follow. 



THE CELL 1 



You are aware that all organisms consist of organs and 

 that each of their organs has a different function : the brain, 

 the liver, the eyes, the hands are types of organs in animals, 

 as are the leaves and the pistils in plants. 



You are also aware that, except in the lowest organisms, 

 the so-called Protista, all organs are built up of cells. That 

 is a simple fact of observation, and I therefore cannot agree 

 with the common habit of giving to this plain fact the title 

 of cell-" theory." There is nothing theoretical in it ; and, 



1 E. B. Wilson, The Cell in Development and Inheritance, New York, 

 Macmillan, 1896. 



