THE GASTR.EA THEORY AND ITS SUCCESSORS. 8 1 



the sponges, were the leading causes in the evolution of 

 the theory. Haeckel entirely overlooked the presence 

 of the flattened ectodermal layer (first discovered by F. 

 E. Schulze in 1875, and since demonstrated by other 

 investigators in many different groups of sponges), and 

 homologized with the ectoderm of the Coelenterata and 

 of the Gastrula the mesogloea, or rather the mesogloea 

 phis the unobserved ectoderm of the sponges. Tiius a 

 sponge was to him a diploblastic organism, the OlyiitJnts 

 being '^nur eine festsitzende Gastrula." 



A complete exposition of the theory and of the facts 

 upon which it was based is to be found in the Jenaische 

 Zeitschrift.^ The starting point in the line of evolution, 

 according to the theory, was a simple mass of protoplasm 

 destitute of a nucleus, the Monentla, a representative of 

 which is found VixProtaniceba, and the disappearance of the 

 nucleus of the ovum previous to its division to form the 

 polar globules, was considered to be the reproduction of 

 this stage in the individual development. It is probable, 

 however, that the nucleus is represented in Protamoeba 

 by scattered particles of chromatin disseminated through 

 the cytoplasm and not yet aggregated into a definite 

 mass, in which case the Moners, as Haeckel understood 

 them, do not exist. Granting the disappearance of the 

 nucleus in the ovum, the explanation that it is a return 

 to an ancestral condition is most unsatisfactory. The 



^ E, Haeckel. Die Gastrsea-Theorie, die phylogenetische Classification 

 des Thierreichs und die Homolugie der Keimblatter. Jenaische Zeitschr. 

 Bd. viii. 1874. 



E. Haeckel. Die Gastrula und die Eifurchung derThiere. Jen. Zeitschr. 

 Bd. ix. 1875. 



E. Haeckel. Nachtrage zur Gastrsea-Theorie. Jen. Zeitschr. Bd. xi. 

 1877. 



