86 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS 



much yolk, in the development of the majority of animals, 

 Haeckel concluded that it represents the individual's re- 

 capitulation of an ancestral stage. He suggested that the 

 simplest many-celled animal was like a gastrula, and 

 this hypothetical ancestor of all Metazoa he called a 

 gastrcea. The gastrula is, on this view, the individual 

 animal's recapitulation of the ancestral gastraea. Rival 

 suggestions have been made : perhaps the original Metazoa 

 were balls of cells like Volvox (Fig. 54), with a central 

 cavity in which reproductive cells lay ; perhaps they were 

 like the platmla larvae of some Coelentera — two-layered, 

 externally ciliated, oval forms without a mouth. 



Fig. 41. — Embryos — (i) of bird ; (2) of man. — After His. 

 The latter about twenty-seven days old. 



y.s., Yolk-sac ; pi., placenta. 



(3) The idea of recapitulation. — It is a matter of experi- 

 ence that we recapitulate in some measure the history of 

 our ancestors. Embryologists have made this fact very 

 vivid, by showing that the individual animal develops along 

 a path the stations of which correspond to some extent 

 with the steps of ancestral history. 



(i) The simplest animals are single 

 cells (Protozoa). 



(2) The next simplest are balls of 



cells {e.g. Volvox). 



(3) The next simplest are two- 



layered sacs of cells {e.g. 



(i) The first stage of development 

 is a single cell (fertilised 

 ovum). 



(2) The next is a ball of cells 



(blastula or morula). 



(3) The next is a two-layered sac 

 of cells (gastrula). 



Hydra). 



Von Baer, one of the pioneer embryologists, acknow- 

 ledged that, with several very young embryos of higher 



