322 THE EVOLUTION OF THE METAZOA 



According to the old interpretation, the three body layers 

 evolved successively one after the other, each after a long 

 interval of time. This conclusion was reached mainly on the 

 basis of the formative processes that can be observed during 

 ontogeny. Yet on the other hand, these processes show a 

 perplexing variety so that it was impossible to decide which 

 of them represents the most primitive form. Haeckel proposed 

 a special stage which contained no body layers and which 

 he placed before the stage with one body layer. He believed 

 he found support for this theory in the ontogenetic facts. He 

 called this stage a morula. It did not actually suit the whole 

 scheme he proposed and it has not been generally accepted. 

 The one-layered stage which had according to his interpre- 

 tation the form of a hollow sphere w^as given the name of 

 a blastaea, after the ontogenetic blastula; it was thus believed 

 to be recapitulated in the blastula. All the cells of the epithe- 

 lium as the most primitive form of the tissue, are identical. 

 The first differentiation had taken place by way of a separation 

 of the vegetative and of the sexual functions, and, in connec- 

 tion with this, of their bearers. This is how the appearance 

 of the "germ path" was also explained. The zoologists 

 have not been too embarassed by the fact that no blastaea- 

 like form can be found among the Metazoa. Haeckel, however, 

 who found such an absence unpleasant, tried to track down 

 such an animal form in the sea plankton. Thus a description 

 was made of a certain "Magosphaera" which, however, had 

 soon to be removed from the list of animal species because 

 it was found to be nothing other than a colony of Flagellata. 

 After this failure a blastaea-like form was borrowed from 

 the plant world; this is the now famous Volvox ; Volvox still 

 continues to play the same role even in spite of the fact that we 

 know that it has nothing in common with the origin of the 

 Eumetazoa. It should serve as a pattern. Volvox is certainly a 

 genuine green alga, and a particularly specialized fresh water 

 alga. It emerged as a final product of evolution along a side- 

 line. Many forms can be found among algae which show a 



