SEGMENTATION 65 



In some respects perhaps the best models of living organisms 

 yet made are the "osmotic growths" produced by Leduc. 1 

 These curious structures were formed by placing a fragment of 

 a salt, for instance calcium chloride, in a solution of some col- 

 loidal substance. As the solid takes up water from the solution 

 a permeable pellicle or membrane is formed around it. The ves- 

 icle thus enclosed grows by further absorption of water, often 

 extending in a linear direction, and in many examples this growth 

 occurs by a series of rhythmically interrupted extensions. Some 

 of the growths thus formed are remarkably like organic structures, 

 and might pass for a series of antennary segments or many other 

 organs consisting of a linear series of repeated parts. In admitting 

 the essential resemblance between these "osmotic growths" 

 and living bodies or their organs I lay less stress on the general con- 

 formation of the growths, which often as Leduc points out, recall 

 the forms of fungi or hydroids, but rather on the fact that the 

 interruptions in the development of these systems are so closely 

 analogous to the segmentations or repetitions of parts character- 

 istic of living things (Fig. 9). In the same way I am less im- 

 pressed by Leduc's models of Karyokinesis, wonderful as they 

 nevertheless are, for the division is here imitated by putting 

 separate drops on the gelatine film. What we most want to know 

 is how in the living creature one drop becomes two. The models 

 of linear segmentation have the remarkable merit that they do in 

 some measure imitate the process of actual division or repetition. 

 So in a somewhat modified method Leduc, by causing the diffusion 

 of a solution in a gelatine film, produced rhythmical or periodic 

 precipitations strikingly reminiscent of various organic tissues, 

 for here also the process of periodic repetition is imitated with 

 success. 



It is a feature common to these and to all other rhythmical 

 repetitions produced by purely mechanical forces that there 

 is resemblance between the members of the series, and that this 

 similarity of conformation may be maintained in most complex 

 detail. When however in the mechanical series some of the 

 members differ from the rest we have no difficulty in recognising 



1 Stephane Leduc, Theorie Physico-Chymique de la Vie, Paris, 1910. 

 6 



