DEVELOPMENT. 19 



sues being formed at the expense of the more superficial pro- 

 toplasm of the cells ; and that, when nucleated cells divide, 

 the division of the nucleus, as a rule, precedes that of the 

 whole cell. 



2. In the course of its development every cell proceeds, 

 from a condition in which it closely resembles every other 

 cell, through a series of stages of gradually-increasing diver- 

 gence, until it reaches that condition in which it presents the 

 characteristic features of the elements of a special tissue. 

 The development of the cell is, therefore, a gradual progress 

 from the general to the special state. 



The like holds good of the development of the body as a 

 whole. However complicated one of the higher animals or 

 plants may be, it begins its separate existence under the 

 form of a nucleated cell. This, by division, becomes con- 

 verted into an aggregate of nucleated cells — the parts of this 

 aggregate, following ditierent laws of growth and multiplica- 

 tion, give rise to the rudiments of the organs ; and the parts 

 of these rudiments again take on those modes of growth, mul- 

 tiplication, and metamorphosis, which are needful to convert 

 the rudiment into the perfect structure. 



The development of the organism as a whole, therefore, 

 repeats in principle the development of the cell. It is a prog- 

 ress from a general to a special form, resulting from the grad- 

 ual differentiation of the primitively similar morphological 

 units of which the body is composed. 



Moreover, when the stages of development of two animals 

 are compared, the number of these stages which are similar 

 to one another is, as a general rule, proportional to the close- 

 ness of the resemblance of the adult forms ; whence it fol- 

 lows that the more closely any two animals are allied in adult 

 structure, the later are their embryonic conditions distinguish- 

 able. And this general rule holds for plants no less than for 

 animals. 



The broad principle, that the form in which the more com- 

 plex living things commence their development is always the 

 same, was first expressed by Harvey in his famous aphorism, 

 " Omne viviwi ex ovo,''^ which was intended simply as a mor- 

 phological generalization, and in no wise implied the rejection 

 of spontaneous generation, as it is commonly supposed to do. 

 Moreover, Harvey's study of the development of the chick led 

 him to promulgate that theory of "epigenesis," in which the 

 doctrine that development is a progress from the general to 

 the special is implicitly contained. 



