ix ONTOGENETIC EVOLUTION: CELL-DIVISION 487 



lead an independent existence, while in the M'etazoa the subdivision 

 is less complete and the growing mass of living substance continues 

 to exist as a coherent individual. The physiological advantages of 

 subdivision of the individual body into cellular units is apparent. 

 It renders possible the intercellular deposit of rigid skeletal materials 

 which act as a support to the organism as a whole : it facilitates 

 localization of function and enables blocks of units specialized for 

 particular functions to be transferred during ontogeny to the positions 

 in which they will be most useful : it enables other units to move 

 hither and thither, either by their own activity or by being swept 

 along in a circulating stream of fluid, to wherever they are specially 

 needed in the course of the ordinary vital processes : and it is of 

 enormous importance in relation to attacks upon the organism from 

 without, whether by limiting the area of injury to comparatively 

 small tracts of living substance or by enabling portions of the living 

 substance specialized for defence to be mobilized and ready to concen- 

 trate at the point of attack. 



Modern science impresses upon us the importance of regarding 

 the individual not merely as an aggregation of cells and organs, but 

 rather as a mass of living substance imperfectly subdivided up into 

 cells and organs : imperfectly because each cell and each organ is 

 inextricably linked up in the living activity of the whole individual. 

 It brings to our notice numerous tissues in which the actual living 

 substance of the constituent cells is linked up by intercellular bridges 

 of protoplasm : it tells us of particular cases of developing embryos 

 where similar intercellular continuity is apparent. The question is 

 thus raised : are we correct in our belief that actual complete separa- 

 tion of cells takes place as a general rule when they undergo fission 

 during ontogeny? More especially is it really the case that the 

 individual blastomeres of the segmenting egg become completely 

 separated from one another : is it not rather the case that the 

 apparently complete separation is only apparent, that the individual 

 blastomeres remain continuous through fine protoplasmic bridges : 

 and that cases of intercellular continuity observed in the adult are 

 merely expressions of the fact that such bridges persist throughout 

 the whole period of development ? 



That the latter is really the case has been held by various 

 workers and supported particularly strongly by Sedgwick (1895, 

 1896). It will however have been gathered by the reader from 

 Chapter I. that such a view is in the opinion of the present writer 

 not tenable. The fact that the blastomeres of a segmenting 

 egg tend to take a spherical form, or at least to be bounded 

 by convex surfaces, seems by the ordinary laws of surface tension 

 to indicate that these blastomeres are not continuous with 

 one another. Continuity of substance between the cells of the 

 embryo or adult is therefore when it occurs a secondary and not a 

 primary phenomenon. At the same time the present writer's 

 observations lead him to agree with Sedgwick that such intercellular 



