CENERALISA TIONS. 



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. 43), with a central cavity in which repro- 

 ductive cells lay ; perhaps they were like the planula larvae 

 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 



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 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). 



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 

 is narrowed. The rabbit begins like a Protozoon as a 

 single cell; after a while it may be compared to the 

 young stage of a very simple vertebrate; afterwards, to 

 the young stage of a reptile; afterwards, to the young 

 stage of almost any mammal ; afterwards, to the young 

 stage of almost any rodent; eventually it becomes un- 

 mistakably a young rabbit. 



Herbert Spencer expressed the same idea, by saying that 

 the progress of development is from homogeneous to 

 heterogeneous, through steps in which the individual 

 history is parallel to that of the race. But Haeckel has 

 illustrated the idea more vividly, and summed it up more 

 tersely, than any other naturalist. His "fundamental 



