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larvae。 If； however； they are left in the sea…water containing the saponin they undergo； a few minutes after membrane…formation； the disintegration known in pathology as CYTOLYSIS。 Membrane…formation is； therefore； caused by a superficial or incomplete cytolysis。 The writer believes that the subsequent treatment of the egg with hypertonic sea…water is needed only to overcome the destructive effects of this partial cytolysis。 The full reasons for this belief cannot be given in a short essay。
Many pathologists assume that haemolysis or cytolysis is due to a liquefaction of certain fatty or fat…like compounds； the so…called lipoids； in the cell。 If this view is correct； it would be necessary to ascribe the fertilisation of the egg to the same process。
The analogy between haemolysis and fertilisation throws； possibly； some light on a curious observation。 It is well known that the blood corpuscles； as a rule； undergo cytolysis if injected into the blood of an animal which belongs to a different family。 The writer found last year that the blood of mammals； e。g。 the rabbit； pig； and cattle； causes the egg of Strongylocentrotus to form a typical fertilisation…membrane。 If such eggs are afterwards treated for a short period with hypertonic sea…water they develop into normal larvae （plutei）。 Some substance contained in the blood causes； presumably； a superficial cytolysis of the egg and thus starts its development。
We can also cause the development of the sea…urchin egg without membrane… formation。 The early experiments of the writer were done in this way and many experimenters still use such methods。 It is probable that in this case the mechanism of fertilisation is essentially the same as in the case where the membrane…formation is brought about； with this difference only； that the cytolytic effect is less when no fertilisation…membrane is formed。 This inference is corroborated by observations on the fertilisation of the sea…urchin egg with ox blood。 It very frequently happens that not all of the eggs form membranes in this process。 Those eggs which form membranes begin to develop； but perish if they are not treated with hypertonic sea… water。 Some of the other eggs； however； which do not form membranes； develop directly into normal larvae without any treatment with hypertonic sea…water； provided they are exposed to the blood for only a few minutes。 Presumably some blood enters the eggs and causes the cytolytic effects in a less degree than is necessary for membrane…formation； but in a sufficient degree to cause their development。 The slightness of the cytolytic effect allows the egg to develop without treatment with hypertonic sea…water。
Since the entrance of the spermatozoon causes that degree of cytolysis which leads to membrane…formation； it is probable that； in addition to the cytolytic or membrane…forming substance （presumably a higher fatty acid）； it carries another substance into the egg which counteracts the deleterious cytolytic effects underlying membrane…formation。
The question may be raised whether the larvae produced by artificial parthenogenesis can reach the mature stage。 This question may be answered in the affirmative； since Delage has succeeded in raising several parthenogenetic sea…urchin larvae beyond the metamorphosis into the adult stage and since in all the experiments made by the writer the parthenogenetic plutei lived as long as the plutei produced from fertilised eggs。
（c）。 ON THE PRODUCTION OF TWINS FROM ONE EGG THROUGH A CHANGE IN THE CHEMICAL CONSTITUTION OF THE SEA…WATER。
The reader is probably familiar with the fact that there exist two different types of human twins。 In the one type the twins differ as much as two children of the same parents born at different periods； they may or may not have the same sex。 In the second type the twins have invariably the same sex and resemble each other most closely。 Twins of the latter type are produced from the same egg； while twins of the former type are produced from two different eggs。
The experiments of Driesch and others have taught us that twins originate from one egg in this manner； namely； that the first two cells into which the egg divides after fertilisation become separated from each other。 This separation can be brought about by a change in the chemical constitution of the sea…water。 Herbst observed that if the fertilised eggs of the sea… urchin are put into sea…water which is freed from calcium； the cells into which the egg divides have a tendency to fall apart。 Driesch afterwards noticed that eggs of the sea…urchin treated with sea…water which is free from lime have a tendency to give rise to twins。 The writer has recently found that twins can be produced not only by the absence of lime； but also through the absence of sodium or of potassium； in other words； through the absence of one or two of the three important metals in the sea…water。 There is； however； a second condition； namely； that the solution used for the production of twins must have a neutral or at least not an alkaline reaction。
The procedure for the production of twins in the sea…urchin egg consists simply in this：the eggs are fertilised as usual in normal sea…water and then； after repeated washing in a neutral solution of sodium chloride （of the concentration of the sea…water）； are placed in a neutral mixture of potassium chloride and calcium chloride； or of sodium chloride and potassium chloride； or of sodium chloride and calcium chloride； or of sodium chloride and magnesium chloride。 The eggs must remain in this solution until half an hour or an hour after they have reached the two…cell stage。 They are then transferred into normal sea…water and allowed to develop。 From 50 to 90 per cent of the eggs of Strongylocentrotus purpuratus treated in this manner may develop into twins。 These twins may remain separate or grow partially together and form double monsters； or heal together so completely that only slight or even no imperfections indicate that the individual started its career as a pair of twins。 It is also possible to control the tendency of such twins to grow together by a change in the constitution of the sea…water。 If we use as a twin…producing solution a mixture of sodium； magnesium and potassium chlorides （in the proportion in which these salts exist in the sea…water） the tendency of the twins to grow together is much more pronounced than if we use simply a mixture of sodium chloride and magnesium chloride。
The mechanism of the origin of twins； as the result of altering the composition of the sea…water； is revealed by observation of the first segmentation of the egg in these solutions。 This cell…division is modified in a way which leads to a separation of the first two cells。 If the egg is afterwards transferred back into normal sea…water； each of these two cells develops into an independent embryo。 Since normal sea…water contains all three metals； sodium； calcium； and potassium； and since it has besides an alkaline reaction； we perceive the reason why twins are not normally produced from one egg。 These experiments suggest the possibility of a chemical cause for the origin of twins from one egg or of double monstros