CORRELATION OF GROWTH. 179 



mirror, it will appear in the mirror often with all 

 the peculiarities of the writer's signature. The 

 significance of this fact is appreciated when we 

 remember the long and difficult practice which was 

 necessary before the right hand attained its present 

 power of writing, and how profoundly this practice 

 has, unknown to us, affected the left hand. 



There is still another kind of correlation of parts 

 which is, not like what we have discussed, depend- 

 ent upon the similarity of form, but rather is de- 

 pendent upon a similarity in the origin of the tissues 

 of the correlated parts. In every species of multi- 

 cellular animals, the egg, or fertilised germ-cell, 

 divides into a number of parts, each forming a cell ; 

 this division is repeated many times, forming thus 

 a cluster of cells which gradually arrange themselves 

 around a central cavity in two or three layers, one 

 inside the other, generally in a spherical or oval 

 shape. The outer layer is called the " epiblast," the 

 inner is called the " hypoblast " ; the middle layer, 

 called the " mesoblast," is secondary, being an out- 

 growth of the hypoblast. In all embryos the epi- 

 blast produces the skin and nervous system, while 

 the hypoblast and its product, the mesoblast, pro- 

 duce respectively the digestive and muscular sys- 

 tems. In the group of Ascidians it is very common, 

 as we have seen, for embryos which have reached 



