132 



Organisms from Eggs 



FIG. 12 



The next divisions bring about an unequal division 

 of the material. Four cells will be formed of ectoderm 



material C and only 

 little intestine ma- 

 terial B, the other 

 four cells containing 

 B and A. These lat- 

 ter form at the next 

 division four very 

 small colourless 

 cells, the so-called 

 micromeres, A (Fig. 

 12), from which the 

 mesenchyme, skele- 

 ton, and connective tissue are formed, four larger cells, 



B, from which the intestine is formed, and eight cells, 



C, from which the ectoderm will arise. The separa- 

 tion of the three groups of substances is probably not 

 as complete as our purely diagrammatic drawing (Fig. 

 12) indicates. 



The cell division proceeds and the cells become smaller 

 and smaller and all gather at the surface of the egg, 

 thus forming a hollow sphere. It is not known what 

 brings about this gathering of the cells at the surface, 

 whether it is protoplasmic creeping or streaming or 

 whether the cells are held by a jelly-like layer which 

 covers the surface of the egg (hyaline membrane) (Fig. 

 13). Then the cilia are formed at the external surface 



