188 L. V. HEILBRUNN. 



cording to Loeb is considerably above that sufficient to check 

 oxidations. 



Loeb also found that the presence of KCN prevented degener- 

 ative change in eggs exposed to hypertonic sea-water, and 

 assumed that the KCN acted by retarding excessive oxidations. 

 However on the basis of our knowledge concerning the anti- 

 swelling effect of KCN (cf. p. 174), it is simpler to assume that 

 the inhibition of membrane swelling is the prime cause in pre- 

 venting disintegration by the hypertonic solution. 



The main support of the oxidation theory no doubt lies in 

 the actual measurements of oxidations made by Warburg and 

 Loeb. The method by which these measurements were made 

 has been criticized in a note in Science (Heilbrunn '15). 



C. An Analysis of the Methods of Producing Segmentation in 

 the Unfertilized Arbacia Egg. 



Hitherto in the study of artificial parthenogenesis, the general 

 tendency has been to find new and different methods, rather 

 than to find points of resemblance in the various means employed 

 to produce segmentation in any one egg. The result has been 

 that too much emphasis has been placed on the diversity and 

 the unrelated character of the numerous parthenogenetic agents. 



As a matter of fact there are in general only two ways of pro- 

 ducing segmentation in the Arbacia egg, the endosmotic method 

 and the exosmotic method. Whenever a reagent lowers the 

 surface tension of the plasma-membrane, endosmosis is the re- 

 sult. The theoretical reason for this has already been considered 

 (cf. p. 1 68). All those reagents which induce either true membrane 

 elevation or membrane swelling are thus included in this cate- 

 gory. In the first case the reagent produces a lowered surface- 

 tension directly, in the second case, the swelling of the membrane 

 results in a lowering of tension. Practically all of the reagents 

 which have been used in artificial parthenogenesis produce either 

 the one type of cortical change or the other. As a result, endos- 

 mosis follows, unless the eggs are in a hypertonic solution. In 

 the latter case, water is extracted from the cell, and exosmosis 

 occurs. 



An apparent exception is found in the action of cold, which 



