560 THE FORMS OF TISSUES [ch. 



phenomena. The kindred but more compUcated phenomena analogous 

 to the polar furrow, which arise 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 less capable 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 over- 

 powered and cancelled by agencies of a different order. Here we 

 pass into a region where observation and experiment 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 always remember that even in a froth of soap-bubbles 

 we can apply an exact analysis only to the simplest cases and 

 conditions; 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 con- 

 sists; but our knowledge is enough to leave us assured that what 

 we have learned by investigation 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 

 egg, we may take brief note of two associated problems: viz. 

 (1) the formation and enlargement of the segmentation cavity, or 

 central interspace around which the cells tend to 'group themselves 

 in a single layer, and (2) the formation of the gastrula, that is to 

 say (in a typical case) the conversion by "invagination," of the 



