SECT. 6] OF THE EMBRYO 853 



use than the micro-electrodes, because so many egg-cells are loaded 

 with yolk and opaque substances, and so few possess a pigment 

 which is a natural indicator, 



A certain number of experiments have been made in which the 

 effect on development of changing the pH of the external medium 

 has been studied. Vies, Dragoiu & Rose, following again their con- 

 ception of "travail d'arret", determined the j^H necessary for com- 

 plete cessation of cleavage in echinoderm eggs. In the case of 

 Strongplocentrotus lividus the percentage of eggs having accomplished 

 the cleavage in question remained high and constant until />H 5-2 was 

 reached, but between 5-2 and 4-9 it decreased by almost 100 per cent. 

 A second paper by Dragoiu, Vies & Rose studied the incidence of 

 cytological abnormalities in the eggs submitted to abnormal hydro- 

 gen ion concentrations, establishing the expected result that the more 

 abnormal the pH the more rapidly the abnormahties occurred. Here 

 again, complete curves on a graph were plotted from only two points in 

 each case. Now, although no effect on the number of cleavages in a given 

 time had been observed between the normal /?H (8-4) and 5-2, it was 

 possible that a small effect had been produced. Labbe argued that this 

 would be better shown over a longer period, so he determined the time 

 taken to reach a definite stage in some polychaete worm embryos, 

 obtaining figures as follows : ^. , - • . 



^ ^ Time taken to reach a given stage 



in hours from fertiHsation at 



pH 8-4 (normal) pH 8-1 



Sabellaria alveolata ... ... ... 27 45 



Halosydna gelatinosa ... ... 18 28 



S . alveolata y. H . gelatinosa ... ... 19 3° 



Evidently a comparatively small change in /?H shows a marked effect 

 on developmental time over a long period. 



Clowes & Smith and Smith & Clowes, in a series of interesting 

 papers, studied the effect of j&H on various factors in development, 

 such as the ageing of unfertilised Arbacia, Asterias and Chaetopterus 

 eggs, the artificial activation of Chaetopterus eggs, and the develop- 

 ment of normally fertiHsed Arbacia and Asterias eggs. Loeb's early 

 work with Arbacia eggs had shown that addition of acid to sea water 

 was always retarding in its action, but that small amounts of alkaH 

 exerted an accelerating action. This accelerating action was not 

 operative before the formation of the blastula, but only between 

 that stage and the stage of the pluteus. Excessive amounts of alkali 



