MONOPHYLETISM 291 



which possess both structures, is for the first time clearly 

 brought out. The derivation of all the Metazoa from a single 

 ancestral form, the Gastraea, leads to the conclusion that the 

 types are not distinct from one another as Cuvier and von 

 Baer supposed, but agree in the one essential point, in the 

 possession of an arcJienteron (Lankester, 1875), an d an 

 ectoderm and endoderm which are homologous throughout 

 all the Metazoan phyla. Finally, in the separation of the 

 sponges, Ccelenterata and Accelomi as animals lacking a body 

 cavity or coelom x from the four higher phyla, which are 

 essentially Ccelomati, there is contained the germ of a 

 conception which later became of importance. 



Somewhat similar views as to the importance of the 

 germ-layer theory for the phylogenetic classification of 

 animals were published by Sir E. Ray Lankester in iS/3. 2 

 He distinguished three grades of animals the Homoblastica, 

 Diploblastica, and Triploblastica. The first included the 

 Protozoa, the second the Coelenterata, the third the other 

 five phyla, distinguished by the possession of a third layer, 

 the mesoderm, and a " blood-lymph " cavity enclosed therein. 

 He used the germ-layer theory to prove the essential unity 

 of type of all the Triploblastica. 



The Gastraea theory gave point and substance to the 

 biogenetic law, and enabled Haeckel to state much more 

 concretely the parallelism existing between ontogeny and 

 phylogeny. He was able to assert that five primordial 

 stages, each representing a primitive ancestral form, recurred 

 with regularity in the very earliest development of all 

 Metazoa. 3 These were the monerula, cytula, morula, blastula. 

 and gastrula (see Fig. 15). The monerula was the fertilised 

 ovum after the disappearance of the germinal vesicle ; 4 

 it was the equivalent of the primordial anucleate Monera 



1 Term first introduced in Die Kalkschwamme, p. 468, 1872. 



2 "On the Primitive Cell-layers of the Embryo as the Basis of 

 Genealogical Classification of Animals, and on the Origin of Vascular 

 and Lymph Systems," Ann. Mag. Nat. Hist. (4), xi., pp. 321-38, 1873. 



3 First distinguished in Die Kalkschwiimme, i., p. 465. 



4 Even in the 'seventies it was still believed by many that the egg- 

 nucleus disappeared on fertilisation. The true nature of the process was 

 not fully made out till 1875, when O. Hertwig observed the fusion of egg- 

 and sperm-nuclei in Toxopneustes (Morph. Jahrb., i., 1876). 



