ELEMENTARY MORPHOGENESIS 37 



no radial divisions occur, is the most important one in its 

 formation. When 808 blastomeres have come into existence 

 the process of cleavage is finished; a sphere with a wall 

 of cells and an empty interior is the result. That only 

 808 cells are formed, and not, as might be expected, 1024, 

 is due to the fact that the micromeres divide less often 

 than the other elements ; but speaking roughly, of course, 

 we may say that there are ten steps of cleavage-divisions 

 in our form; 1024 being equal to 2 10 . 



We have learned that the first process of development, 

 the cleavage, is carried out by simple cell- division. A few 

 cases are known, in which cell-division during cleavage is 

 accompanied by a specific migration of parts of the 

 protoplasm in the interior of the blastomeres, especially in 

 the first two or first four ; but in almost all instances 

 cleavage is as simple a process of mere division as it is in 

 our sea-urchin. Now the second step in development, at 

 least in our form, is a typical histological performance : it 

 gives a new histological feature to all of the blastomeres : 

 they acquire small cilia on their outer side and with these 

 cilia the young germ is able to swim about after it has 

 left its membrane. The germ may be called a " blastula " 

 at this stage, as it was first called by Haeckel, whose 

 useful denominations of the first embryonic stages may 

 conveniently be applied, even if one does not agree with 

 most, or perhaps almost all, of his speculations (Fig. 2). 



It is important to notice that the formation of the 

 " blastula " from the last cleavage stage is certainly a 

 process of organisation, and may also be called a 

 differentiation with regard to that stage. But there is in 

 the blastula no trace of one part of the germ becoming 



