376 



THE FORMS OF TISSUES 



[CH. 



arrangements are all equally good; some are known to be much 

 more stable than others, and some have never yet been realised 

 in actual experiment. 



The conditions which lead to the presence of any one of them, 

 in preference to another, are as yet, so far as I am aware, un- 

 determined, but to this point we shall return. 



Examples of these various arrangements meet us at every 

 turn, and not only in cell-aggregates, but in various cases where 

 non-rigid and semi-fluid partitions (or partitions that were so to 

 begin with) meet together. And it is a necessary consequence of 

 this physical phenomenon, and of the limited and very small 

 number of possible arrangements, that we get similar appearances, 

 capable of representation by the same diagram, in the most 

 diverse fields of biology*. 



Fig. 159. 



Among the published figures of embryonic stages and other 

 cell aggregates, we only discern these little intermediate partitions 

 in cases where the investigator has drawn carefully just what lay 

 before him, without any preconceived notions as to radial or other 

 symmetry; but even in other cases we can generally recognise, 

 without much difficulty, what the actual arrangement was whereby 

 the cell- walls met together in equilibrium. I have a strong sus- 

 picion that a leaning towards Sachs's Rule, that one cell-wall tends 

 to set itself at right angles to another cell-wall (a rule whose strict 

 limitations, and narrow range of application, we have already 



* Even in a Protozoon (Euglena viridis), when kept alive under artificial com- 

 pression, Ryder found a process of cell-division to occur which he compares to 

 the segmenting blastoderm of a fish's egg, and which corresponds in its essential 

 features with that here described. Confrib. Zool. Lab. Univ. Pennsylvania, i, 

 jDp. 37-50, 1893. 



