36 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



right angles to the first two ; it may be called an equatorial 

 plane, if we compare the egg with a globe ; it also divides 

 equally, and so we now find two rings, each consisting of 

 four cells, and one above the other. But now the cell- 

 divisions cease to be equal, at least in one part of the egg : 

 the next division, which leads from the eight- to the 

 sixteen -cell stage of cleavage, forms four rings, of four 

 cells each, out of the two rings of the eight-cell stage. 

 Only in one half of the germ, which we shall call the upper 

 one, or which we might call, in comparison with a globe, 

 the northern hemisphere, are cells of equal size to be found ; 

 in the lower half of the egg four very small cells have been 

 formed at one " pole " of the whole germ. We call these 

 cells the " micromeres," that is, the " small parts," on the 

 analogy of the term " blastomeres," that is, parts of the germ, 

 which is applied to all the cleavage cells in general. The 

 place occupied by the micromeres is of great importance 

 to the germ as a whole : the first formation of real organs 

 will start from this point later on. It is sufficient thus 

 fully to have studied the cleavage of our Echinus up to 

 this stage : the later cleavage stages may be mentioned 

 more shortly. All the following divisions are into equal 

 parts ; there are no other micromeres formed, though, of 

 course, the cells derived from the micromeres of the sixteen- 

 cell stage always remain smaller than the rest. All the 

 divisions are tangential ; radial cleavages never occur, and 

 therefore the process of cleavage ends at last in the forma- 

 tion of one layer of cells, which forms the surface of a 

 sphere ; it is especially by the rounding-up of each blasto- 

 mere, after its individual appearance, that this real surface 

 layer of cells is formed, but, of course, the condition, that 



