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Miss D. J. Lloyd. Influence of Osmotic 



the sections shown in fig. 8. It can be seen that where a new part is 

 growing, an old part is correspondingly diminishing (fig. 8, 1-4). In 

 cases where there is no formation of new parts (fig. 8, 5-8), there is 

 correspondingly no diminution of the old ones. Thus, when no tail grows 

 out, and where the pharynx also shows no growth, the part anterior to the 

 pharynx remains with unchanged proportions. The value of the irregular 

 mitoses found in the parenchyma cells under the conditions unfavourable to 

 development remains to be considered. Under normal conditions regenera- 

 tion takes place without the parenchyma cells increasing to any great extent 

 in number. For this reason it seems probable that the failure to restore the 

 lost parts under conditions of very high or very low osmotic pressure is not 

 due to failure of the parenchyma cells to divide, but that both checked 

 migration and irregular mitoses are an expression of unfavourable change in 

 the physiological conditions. This suggestion receives support from the 

 appearance presented by the gut cells, which in strongly hyptonic solutions 

 are swollen and vacuolar, while in hypertonic solutions they are contracted 

 and dense. 



The most favourable conditions for the regeneration of G. ulvce are in a 

 mixture of 100 parts of distilled water to 500 of sea-water, and at an 

 •osmotic pressure of 18 atmospheres. Regeneration occurs, though less 

 rapidly, in solutions having an osmotic pressure as low as 7'5, or as high as 

 29 - 5 atmospheres. Above or below these two points the water-content of the 

 tissues appears to have been altered to such an extent that the cells, notably 

 the parenchyma cells, can no longer perform their normal functions. 



Summary. 



1. G. ulvce is capable of living indefinitely in water having an osmotic 

 pressure of more than 2 and less than 33 atmospheres. 



2. The rate of regeneration of the posterior end in G. ulvce depends on the 

 osmotic pressure of the medium. This osmotic pressure has an optimum 

 value for regeneration at 18 atmospheres, i.e., just below that of sea-water, 

 and limiting values at 5 and 33"5 atmospheres. 



3. Restoration of lost parts in G. ulvce is brought about entirely by the 

 undifferentiated parencbyma cells which migrate to the region of the wound 

 and build up the lost parts. 



4. For values of the osmotic pressure lying between the optimum and the 

 limiting values, this migration of the parenchyma cells is retarded, and the 

 rate of restoration is retarded to a similar degree. At the limiting values of 

 the osmotic pressure there is no migration of the parenchyma ' cells and no 

 restoration of lost parts. 



