CHAPTER XII 



THE LIFE-HISTORY OF ANIMALS 



I. Modes of Reproduction 2. Divergent Modes of Reproduction 

 3. Historical 4. The Egg -Cell or Ovum $. The Male- 

 Cell or Spermatozoon 6. Maturation of the Ovum 7. 

 Fertilisation 8. Segmentation and the first stages in 

 Development 9. Some Generalisations The Ovum Theory ', 

 the Gastrcza Theory ', Fact of Recapitulation ^ Organic Con- 

 tinuity 



IN his exercitation " on the efficient cause of the 

 chicken," Harvey (1651) confesses that "although it be a 

 known thing subscribed by all, that the foetus assumes its 

 original and birth from the male and female, and conse- 

 quently that the egge is produced by the cock and henne, 

 and the chicken out of the egge, yet neither the schools of 

 physicians nor Aristotle's discerning brain have disclosed 

 the manner how the cock and his seed doth mint and 

 coine the chicken out of the egge." The marvellous 

 facts of growth are familiar to us the sprouting corn 

 and the opening flowers, the growth of the chick within 

 the egg and of the child within the womb ; yet so 

 difficult is the task of inquiring wisely into this marvel- 

 lous renewal of life that we must reiterate the old 

 confession : " ingratissimum opus scribere ab iis quae, 

 multis a natura circumjectis tenebris velata, sensuum 

 lucis inaccessa, hominum agitantur opinionibus." 



i. Modes of Reproduction. The simplest animals 

 divide into two or into many parts, each of which becomes a 

 full-grown Protozoon. There is no difficulty in understanding 



