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boratory； developed as well and lived as long as the pure breeds of the sea…urchin； and there was nothing to indicate any difference in the vitality of the two breeds。
So far as the question of heredity is concerned； all the experiments on heterogeneous hybridisation of the egg of the sea…urchin with the sperm of starfish； brittle…stars； crinoids and molluscs； have led to the same result； namely； that the larvae have purely maternal characteristics and differ in no way from the pure breed of the form from which the egg is taken。 By way of illustration it may be said that the larvae of the sea… urchin reach on the third day or earlier （according to species and temperature） the so…called pluteus stage； in which they possess a typical skeleton； while neither the larvae of the starfish nor those of the mollusc form a skeleton at the corresponding stage。 It was； therefore； a matter of some interest to find out whether or not the larvae produced by the fertilisation of the sea…urchin egg with the sperm of starfish or mollusc would form the normal and typical pluteus skeleton。 This was invariably the case in the experiments of Godlewski； Kupelwieser； Hagedoorn； and the writer。 These hybrid larvae were exclusively maternal in character。
It might be argued that in the case of heterogeneous hybridisation the sperm…nucleus does not fuse with the egg…nucleus； and that； therefore； the spermatozoon cannot transmit its hereditary substances to the larvae。 But these objections are refuted by Godlewski's experiments； in which he showed definitely that if the egg of the sea…urchin is fertilised with the sperm of a crinoid the fusion of the egg…nucleus and sperm…nucleus takes place in the normal way。 It remains for further experiments to decide what the character of the adult hybrids would be。
（b）。 ARTIFICIAL PARTHENOGENESIS。
Possibly in no other field of Biology has our ability to control life… phenomena by outside conditions been proved to such an extent as in the domain of fertilisation。 The reader knows that the eggs of the overwhelming majority of animals cannot develop unless a spermatozoon enters them。 In this case a living agency is the cause of development and the problem arises whether it is possible to accomplish the same result through the application of well…known physico…chemical agencies。 This is； indeed； true； and during the last ten years living larvae have been produced by chemical agencies from the unfertilised eggs of sea…urchins； starfish； holothurians and a number of annelids and molluscs； in fact this holds true in regard to the eggs of practically all forms of animals with which such experiments have been tried long enough。 In each form the method of procedure is somewhat different and a long series of experiments is often required before the successful method is found。
The facts of Artificial Parthenogenesis； as the chemical fertilisation of the egg is called； have； perhaps； some bearing on the problem of evolution。 If we wish to form a mental image of the process of evolution we have to reckon with the possibility that parthenogenetic propagation may have preceded sexual reproduction。 This suggests also the possibility that at that period outside forces may have supplied the conditions for the development of the egg which at present the spermatozoon has to supply。 For this； if for no other reason； a brief consideration of the means of artificial parthenogenesis may be of interest to the student of evolution。
It seemed necessary in these experiments to imitate as completely as possible by chemical agencies the effects of the spermatozoon upon the egg。 When a spermatozoon enters the egg of a sea…urchin or certain starfish or annelids； the immediate effect is a characteristic change of the surface of the egg； namely the formation of the so…called membrane of fertilisation。 The writer found that we can produce this membrane in the unfertilised egg by certain acids； especially the monobasic acids of the fatty series； e。g。 formic； acetic； propionic； butyric； etc。 Carbon…dioxide is also very 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 min