606 



THE FORMS OF TISSUES 



[CH. 



We begin to realise the remarkable fact, which may even appear 

 a startHng one to the biologist, that all possible groupings or 

 arrangements whatsoever of eight cells in a single layer or surface 

 (none being submerged or wholly enveloped by the rest) are referable 

 to one or another of twelve types or patterns; and that all the 

 thousands and thousands of drawings which dihgent observers have 

 made of such eight-celled embryos or blastoderms, or other eight- 

 celled structures, animal or vegetable, anatomical, histological or 



Fig. 257. Photographs of lVf)<fs' eggs, slunving various arrangements, or 

 partitionings, of the lirst eight cells. 



embryological, are one and all of them representations of some one 

 or other of these twelve types— or rather for the most part of less 

 than the whole twelve; for a certain small number are essentially 

 unstable, and have at best but a transitory and evanescent existence. 

 But that even the unstable cases should now and then be seen is 

 not to be wondered at : when viscidity and friction, and in general 

 the imperfect fluidity of the system, retard the adjustment of the cells 

 and delay the advent of equilibrium. 



As soon as we reahse that the number of cell-patterns, for instance 

 in a segmenting egg, is strictly limited, we want to know how many 



