GENERALISATIONS. 69 



Generalisations (i) The Ovum Theory or Cell Theory. 

 All many celled animals, produced by sexual reproduction, 

 begin at' the beginning again. "The Metazoa begin where 

 the Protozoa leave off" as single cells. Fertilisation does 

 not make the egg cell double ; there is only a more com- 

 plex and more vital nucleus than before. All development 

 takes place by the division of this fertilised egg cell and its 

 descendant cells. 



(2) The Gastrcea 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, H?eckel 

 concluded that it represents the individual's recapitulation 

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



pi. 



FIG. 14. Embryos (i) of bird ; (2) of man. (After His.) 

 The latter about twenty-seven days old. 



y.s. Yolk sac ; //. placenta. 



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

 thetical ancestor of all Metazoa he calls a gastr&a. The 

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

 tion of the ancestral gastrasa. Rival suggestions have been 

 made : perhaps the original Metazoa were balls of cells like 

 Volvox, with a central cavity in which reproductive cells 

 lay ; perhaps they were like the planula larvae of some 

 Ccelenterates 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 



