CHAPTEK XVIII 



Ontogeny The recapitulation hypothesis Interpretation of the onto- 

 genetic record Palingeiietic and csenogenetic characters. 



IN dealing with the cell theory, in Chapter IV, we have 

 already laid stress upon the fact that every multicellular animal 

 or plant commences its individual life as a unicellular egg or 

 ovum, and gradually passes by slow stages of cell-division and 

 differentiation into the adult condition. There are, of course, 

 apparent exceptions to this rule in the case of animals and 

 plants which reproduce by budding or by some analogous process, 

 where the bud arises from a group of cells belonging to the 

 parent, but even in these cases reproduction by means of 

 germ cells is resorted to at more or less frequent intervals and 

 the budding must be regarded merely as an additional method of 

 multiplication interpolated in the life-cycle. 



The individual organism, then, does not come into existence 

 fully formed in all its perfection, but passes through a longer or 

 shorter process of development to the adult condition. In fact 

 it undergoes a process of individual evolution which constitutes 

 its individual life-history or ontogeny, and the length and 

 complexity of this process are proportional to the complexity of 

 organization of the adult. Thus the complete life-cycle of many 

 of the lower multicellular animals and plants is passed through 

 in the course of a few months, while a man requires twenty 

 years or more to attain his full development. 



Although, for the sake of convenience, the ontogeny of any 

 given organism may be divided up into a number of stages, yet 

 these stages cannot in reality be sharply distinguished from 

 one another. Even the division or segmentation of the ovum 

 (Figs. 13 and 119), whereby the organism passes from the uni- 

 cellular to the multicellular condition, does not take place 

 suddenly but by means of the slow and complicated process of 

 mitosis or karyokinesis (Fig. 31) ; and in cases where there is an 

 apparently abrupt change, or metamorphosis, from one condition 



