darwin and modern science-第89章
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ery efficient in this direction。 It was also found that the higher acids are more efficient than the lower ones； and it is possible that the spermatozoon induces membrane…formation by carrying into the egg a higher fatty acid； namely oleic acid or one of its salts or esters。
The physico…chemical process which underlies the formation of the membrane seems to be the cause of the development of the egg。 In all cases in which the unfertilised egg has been treated in such a way as to cause it to form a membrane it begins to develop。 For the eggs of certain animals membrane… formation is all that is required to induce a complete development of the unfertilised egg； e。g。 in the starfish and certain annelids。 For the eggs of other animals a second treatment is necessary； presumably to overcome some of the injurious effects of acid treatment。 Thus the unfertilised eggs of the sea…urchin Strongylocentrotus purpuratus of the Californian coast begin to develop when membrane…formation has been induced by treatment with a fatty acid； e。g。 butyric acid； but the development soon ceases and the eggs perish in the early stages of segmentation； or after the first nuclear division。 But if we treat the same eggs； after membrane… formation； for from 35 to 55 minutes （at 15 deg C。） with sea…water the concentration （osmotic pressure） of which has been raised through the addition of a definite amount of some salt or sugar； the eggs will segment and develop normally； when transferred back to normal sea…water。 If care is taken； practically all the eggs can be caused to develop into plutei； the majority of which may be perfectly normal and may live as long as larvae produced from eggs fertilised with sperm。
It is obvious that the sea…urchin egg is injured in the process of membrane…formation and that the subsequent treatment with a hypertonic solution only acts as a remedy。 The nature of this injury became clear when it was discovered that all the agencies which cause haemolysis； i。e。 the destruction of the red blood corpuscles； also cause membrane…formation in unfertilised eggs； e。g。 fatty acids or ether； alcohols or chloroform； etc。； or saponin； solanin； digitalin； bile salts and alkali。 It thus happens that the phenomena of artificial parthenogenesis are linked together with the phenomena of haemolysis which at present play so important a role in the study of immunity。 The difference between cytolysis （or haemolysis） and fertilisation seems to be this； that the latter is caused by a superficial or slight cytolysis of the egg； while if the cytolytic agencies have time to act on the whole egg the latter is completely destroyed。 If we put unfertilised eggs of a sea…urchin into sea…water which contains a trace of saponin we notice that； after a few minutes； all the eggs form the typical membrane of fertilisation。 If the eggs are then taken out of the saponin solution； freed from all traces of saponin by repeated washing in normal sea…water； and transferred to the hypertonic sea…water for from 35 to 55 minutes； they develop into 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 fert
