42 GENERAL EMBRYOLOGY 



see later, this fact of polarity becomes of the greatest importance, 

 for it is frequently related closely to the symmetry of the mature 

 organism. 



While we may thus describe the Metazoan tissue cell as a 

 separate morphological unit, complete in itself, it is also true 

 that in a great many tissues the cells are in direct material 

 continuity with one another through minute protoplasmic 

 connections or bridges which pass through fine perforations in 

 the cell walls. These have been observed in a great variety 

 of tissues in many different forms (Fig. 14, X) ; whether or not 

 they are present in all tissues it is yet impossible to say definitely, 

 and we must recognize clearly that in the physiology of the 

 organism the cells do not behave as completely autonomic 

 units. While each represents a localized field of activity, the 

 life of the cell is subordinated to the life of the organism as a 

 whole a fact that comes out with especial clearness in the 

 development of the organism. In some way not yet understood 

 the cell, or groups of cells, influence the activities of other cells, 

 and are in turn influenced by them. The activities of the 

 Metazoan organism of course equal the sum of the activities of 

 its component cells; but these combined activities are organized 

 and unified into a whole in such a way that this represents 

 more than the unit activities when considered separately, just 

 as the action of a community represents something beyond 

 the sum of the actions of its members taken individually. 



From the embryological point of view one of the most im- 

 portant phases of cell activity is cell reproduction or cell 

 division. For cells arise only from preexisting cells. The 

 history of opinion regarding the genesis of cells parallels roughly 

 that regarding the genesis of organisms. It was Virchow who 

 finally demonstrated convincingly (1855, 1858) the universality 

 of the fact of cell continuity in the tissues of a single organism, 

 and further the fact that in a succession of generations of 

 organisms the process of the formation of cells from preexisting 

 cells is not interrupted. We know now that in this process of 

 cell division all the essential organs of the cell take an active 



