70 REPRODUCTION AND LIFE HISTORY. 



Physiological embryology. Of the physiological conditions of develop- 

 ment we know relatively little. To investigate them is one of the 

 tasks of the future. Why does the fertilised egg-cell divide, how does 

 the yolk affect segmentation, what are the conditions of the infolding 

 which forms the endoderm, and of the outfolding which makes the 

 ccelom pouches ; in short, what are the immediate conditions of each 

 step in the familiar process by which out of apparent simplicity obvious 

 complexity arises? 



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 



v.s. 



FIG. 34. Embryos (i) of bird ; (2) of man. After His. 

 The latter about twenty-seven days old. 



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



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 

 descendent cells. 



(2) The gastric a 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 gastrffa. The 

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

 tion of the ancestral gastraea. Rival suggestions have been 

 made : perhaps the original Metazoa were balls of cells like 

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



