vii] OF POLAR FURROWS 343 



ates, vertebrates and what not, we meet with now one and now 

 another, in a medley which defies classification. They are not 

 "vital phenomena," or "functions" of the organism, or special 

 characteristics of this or that organism, but purely physical 

 phenomena. The kindred but more complicated phenomena 

 which correspond to the polar furrow when a larger number of 

 cells than four are associated together, we shall deal with in the 

 next chapter. 



Having shewn that the capillary phenomena are patent and 

 unmistakable during the earlier stages of embryonic development, 

 but soon become more obscure and incapable of experimental 

 reproduction in the later stages, when the cells have increased in 

 number, various writers including Robert himself have been 

 inclined to argue that the physical phenomena die away, and are 

 overpowered and cancelled by agencies of a very different order. 

 Here we pass into a region where direct observation and experi- 

 ment are not at hand to guide us, and where a man's trend of 

 thought, and way of judging the whole evidence in the case, must 

 shape his philosophy. We must remember that, even in a froth 

 of soap-bubbles, we can apply an exact analysis only to the simplest 

 cases and conditions of the phenomenon ; we cannot describe, 

 but can only imagine, the forces which in such a froth control the 

 respective sizes, positions and curvatures of the innumerable 

 bubbles and films of which it consists ; but our knowledge is 

 enough to leave us assured that what we have learned by in- 

 vestigation of the simplest cases includes the principles which 

 determine the most complex. In the case of the growing embryo 

 we know from the beginning that surface tension is only one of 

 the physical forces at work; and that other forces, including 

 those displayed within the interior of each living cell, play their 

 part in the determination of the system. But we have no evidence 

 whatsoever that at this point, or that point, or at any, the dominion 

 of the physical forces over the material system gives place to a 

 new condition where agencies at present unknown to the physicist 

 impose themselves on the living matter, and become responsible 

 for the conformation of its material fabric. 



Before we leave for the present the subject of the segmenting 



