Embryology. 133 



doctrine on embryology, we may refer to Haeckel's 

 Gastrcea Theory (1874). Here we have to The Gastraea 

 distinguish between the observational basis Theory. 

 and the inference drawn from it. The observational 

 basis consisted in showing 1 that one of the most frequent 

 embryonic stages in animals is a two-layered sac, the 

 "gastrula"; it is very clearly seen in the development 

 of sponge, star-fish, earth-worm, pond-snail, lancelet, 

 and so on; in other cases its occurrence is disguised by 

 the presence of a large quantity of yolk ; in some other 

 cases, e.g. mammals, it must be allowed that the gas- 

 trula is far to seek. At the same time it is certain that 

 the gastrula is a very common embryonic stage, and 

 Haeckel drew the inference that the ancestral form of 

 multicellular animals was like a gastrula. He called 

 this hypothetical ancestral type the Gastraea. For many 

 years this theory was the centre of lively and fruitful 

 discussion. 



The broadest generalization which has yet come from 

 embryology is known as the Recapitulation Doctrine or 

 biogenetic law, which expresses the con- The Re 

 elusion that the individual development is in capitulation 

 some measure a recapitulation of the racial 

 history. The theory is an outcome of the mutual in- 

 fluence of evolution-theory and embryology. 



In 1821 Meckel directed attention to the close simi- 

 larity of the early embryonic stages in quite different 

 animals, and spoke of "a correspondence between the 

 development of the embryo and that of the entire animal 

 series". The idea was also familiar to Oken, who gave 

 it evolutionary significance, and did much to introduce 

 it into biology. 



Von Baer remarked on the close resemblances between 

 the embryos of animals the adult forms of which are 

 very different; a reptile-embryo, a bird-embryo, and a 

 mammal-embryo are at certain stages very similar, and 

 the illustrious embryologist confessed that he was un- 

 able to tell to which of these groups three unlabelled 

 embryos before him really belonged. A careful exami- 

 nation of his "laws" shows, however, that he did not 

 accept the recapitulation without many saving clauses. 



