GENERALISATIONS. zr 
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) Zhe gastrea 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 gastrea. The 
gastrula is, on this view, the individual animal’s recapitula- 
tion of the ancestral gastreea. Rival suggestions have been 
made: perhaps the original Metazoa were balls of cells like 
Volvox (Fig. 43), with a central cavity in which repro- 
ductive cells lay; perhaps they were like the planula larvie 
of some Ccelentera—two-layered, externally ciliated, oval 
forms without a mouth. 
(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 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 | (1) The first stage of development 
cells (Protozoa). is a single cell (fertilised 
(2) The next simplest are balls of ovum). 
cells (¢.g. Volvox). (2) The next is a ball of cells 
(3) The next simplest are two- (blastula or morula). 
layered sacs of cells (e.g. | (3) The next is a two-layered sac 
Hydra). of cells (gastrula). 
Von Baer, one of the pioneer embryologists, acknow- 
ledged that, with several very young embryos of higher 
Vertebrates before him, he could not tell one from the 
other. Progress in development, he said, was from a 
general to a special type. In its earliest stage every 
organism has a great number of characters in common 
with other organisms in their earliest stages; at each 
successive stage the series of embryos which it resembles 
