5O SECONDARY LAYERS OF BLASTODERMA. 



secondary layers, which constitute the foundation of 

 the different systems of organs of the embryo. 



The cells composing each of these secondary 

 layers are at first, says Haeckel, quite similar, but 

 they soon become differentiated. That the cells of 

 one layer differ from those of another is evident ; 

 but that they were originally homogeneous not only 

 in form but also in endowments and potentialities, 

 and that they spontaneously differentiated themselves, 

 is the very thing to be contested, as I have before 

 shown. True differentiation consists in this : Out 

 of a collection of originally heterogeneous cells, one 

 kind is metamorphosed into one structure, another 

 kind into another, in the further course of develop- 

 ment, by virtue of their own inherent powers, just 

 as, to repeat the illustration, different kinds of crys- 

 tals are deposited from a mixture of the solutions of 

 different kinds of salts. 



The secondary layers of which Ihe blastoderma 

 comes to consist are found not only in the ova of 

 man, the mammifera and other vertebrata, but also 

 in that of mollusca, arthropoda, echinodermata, the 

 higher worms and higher zoophytes a fact in com- 

 parative ontogenesis which Haeckel interprets as 

 being of the highest phylogenetic significance. To 

 another mind, however, the fact only shows what 

 has -been long recognised, that all forms are con- 

 structed according to a common plan. 



The most important phenomena of general sig- 



