SECT. 6] OF THE EMBRYO 855 



a little retarded. Medes studied the morphology of the plutei raised 

 from solutions of varying acidity. Finally Richards observed ac- 

 celeration of the early cleavage stages of the opisthobranch Haminea 

 virescens in sea water to which sodium hydroxide or potassium 

 hydroxide had been added. None of these workers had controlled 

 the/>H, so Smith & Clowes undertook to do so, raising these numerous 

 isolated fragments on to a quantitative basis. Their results are shown 

 in Fig. 210, where the percentage development (number of cleavages 

 per tgg) is shown plotted against the pH. of the sea water, normal 

 development atj&H 8-15 being taken as 100. The sharp drop between 

 />H 4-8 and 5-4 equates well with the results of Vies, Dragoiu & Rose. 

 The slight effect of hydrogen ion concentrations above normal in 

 accelerating division is well shown on the curves ; as the /?H is raised 

 it soon gives place to the retarding effect which brings the percent- 

 age development down to zero by the time />H 10 is reached. The 

 striking thing is that the limiting hydrogen ion concentrations are 

 characterised not by a gradual but by an abrupt inhibition of develop- 

 ment, while between them it is essentially unimpaired. These results 

 were afterwards repeated and confirmed by Gellhorn. 



According to Smeleva and McCoy, nematode eggs are inde- 

 pendent of external pH over a very wide range ; an interesting fact 

 in view of their remarkably impermeable membranes (see p. 327). 



6-2. The pH of Terrestrial Eggs 



As regards terrestrial eggs, those of birds have been most investi- 

 gated, but we possess some figures due to Fink for the />H of the egg- 

 contents of some insects, colorimetrically estimated, as follows : 



Colorado potato-beetle {Leptimtarsa decemlineata) 



,, peach-beetle {Cotinis nitida) ... 

 Japanese beetle {Popillia japonica) 

 Squash ladybird {Epilachna borealis) 

 Seedcorn maggot {Hylemyia cilicrura) ... 

 Squash bug {Anasa tristis) 



Thus in some cases there was no change, and in others there was a 

 certain rise towards the alkaline side as development proceeded. 



We may now take up the discussion of the total acidity and the 

 pYi of the various parts of the bird's ^gg, and the changes which it 

 undergoes during the course of development. The classical paper on 



N E II 55 



