68 



REPRODUCTION AND LIFE HISTORY. 



(2) The gastraa theory. — As a two-layered gastrula stage 

 occurs, though sometimes disguised by the presence of much 

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

 concluded that it represents the individual's recapitulation 

 of an ancestral stage. He suggested that the simplest stable, 

 many-celled animal was like a gastrula, and this hypo- 

 thetical ancestor of all Metazoa he called a gastrcea. The 

 gastrula is, on this view, the individual animal's recapitula- 

 tion of the ancestral gastrsea. Rival suggestions have been 

 made : perhaps the original Metazoa were balls of cells like 

 Volvox (Fig. 41), with a central cavity in which repro- 

 ductive cells lay; perhaps they were like the planula larvas 



,)y.s. 



Fig. 31. — Embryos — (1) of bird; (2) of man. — After His. 

 The latter about twenty-seven days old. 



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



of some Coelentera — two-layered, externally ciliated, oval 

 forms without a mouth. 



(3) The fact 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 most 

 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. 



( 1 ) 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. 

 Hydra). 



(1) 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). 



