62 REPRODUCTION AND LU'E-IIlSrORV OF ANIMALS. 



numerous larval stages mean ? Aboui these and many 

 other questions the student should think, 



Generalisations— (\) 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, I''cnilisation does 

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

 and more vital nucleus than before. Moreover, all develop- 

 ment means the division of this fertilised egg-cell and its 

 descendant cells. 



(2) The Gastrcea Theory.— \% a two layered gastrula stage 

 occurs, though sometimes disguised by the jiresence of much 

 yolk, in the development of almost all animals, Iliuckel 

 naturally concluded that it reiiresents the individual's recapit- 

 ulation of an ancestral stage. He believes that the simplest 

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

 hypothetical ancestor of all Metazoa he calls a gastr<ta. The 

 gastrula is the individual animal's rec.apilulation of the 

 ancestral gastrasa. Rival suggestions have 1 leen made ; 

 perhaps the original Mela/oa were balls of cells like Folvox, 

 with a central cavity in which reproductive cells lay ; perhajis 

 they were like the planulaAarvm of some Ctelenterates — 

 two-layered, externally ciliated, oval forms without a mouth. 

 These and other suggestions have been made, but the gastnea 

 theory holds the field. 



The Fact of Recapitulation. — It is a matter (jf exjjerience 

 that we recapitulate in some nie.asure the hi.story of our 

 ancestors. Embryologisls have made this fact most vivid, 

 by showing that the individual animal develops along a path 

 the stations of which rejiresent the stejjs of ancestral history. 



(1) The simplest .animals arc single 



cells. 



(2) The next simplest are balls of 



cells. 



(3) The next simplest arc tWd- 



layered sacs of cells. 



(1) The first slam- of di-vclupmcnt 



is a silicic cell. 



(2) The iicxi is a liall of cells 



(l)lasliilii or morula). 



(3) The ncxl is a two-liiyi-rcil siii' 



of cells (gastrula). 



Von Baer, one of the pioneer emhryologists, acknowledged 

 that with three very young embryos of higher Vertebrates 

 {e.g., rabbit, dog, man, or even reptile, bird, and mammal), 

 JDefore him, he could not tell one from the other. Progress 

 in development, he .said, was from a general to a special 



