The Behavior of Cells 179 



der to discover something of the relations of de- 

 pendence subsisting between the various activi- 

 ties which lead to the formation of the embryo. 



When one watches the early development of the 

 egg of a mollusk or annelid worm, follows the reg- 

 ular and almost mathematically precise way in which 

 cell division occurs, and the method by which the 

 cells become arranged in a perfectly regular pat- 

 tern, then observes the infoldings and overgrowth 

 that lead to the gastrula stage, and the differen- 

 tiation of particular cells to form the organs of the 

 embryo, he cannot escape a feeling of wonder that 

 a bit of simple material, such as the undivided egg 

 appears to be, contains such powers of elaborate 

 and well-ordered construction. A marvellous 

 builder this bit of egg substance ! It is comparatively 

 easy to make a sort of rough catalogue of the meth- 

 ods it employs. Development may be said to be an 

 affair of cell division, cell growth, cell differentia- 

 tion, changes in the form of cells, changes in their 

 position, etc.; but this cataloguing of processes is 

 a mere preliminary to the business of further analy- 

 sis. In order to build up an embryo, cells must di- 

 vide, they must in most cases grow, and they must 

 get into the right relative position and differenti- 

 ate, some in this way and some in that, so as to es- 

 tablish a harmoniously working mechanism. In 

 most organisms formative processes involve a con- 

 siderable amount of cell movement. Sometimes this 

 is passive, as when cells are pushed or pulled by 



