482 LIFE : OUTLINES OF GENERAL BIOLOGY 



ftsh, and even amphibian; and second, that the artificial stimuli 

 effectively used are very varied— chemical, physical, and mechanical. 

 Artificial parthenogenesis has been induced by altering the chemical 

 composition of the water by adding or removing certain salts, or by 

 altering the concentration by adding salt and sugar, or by subjecting 

 the ova to various influences, such as superabundance of carbon 

 dioxide, vapour of chloroform, ether, benzol, and toluol, the presence 

 of butyric acid, blood, serum, and extracts of foreign cells, or by 

 exposing the ova to electric currents, or to mechanical stimulation. 

 Bataillon has sliovvn that frog's eggs pricked with a needle and 

 washed(»with blood may proceed to develop rapidly and normally. 

 In a few cases the parthenogenetic development has been success- 

 fully carried beyond the completion of the tadpole metamor- 

 phosis. The effective stimuli, such as have been enumerated 

 above, differ for different kinds of eggs and even for eggs of the 

 same kind at different stages of ripeness. There is probably some 

 common factor in all the effective stimuli, but what it is remains 

 uncertain. 



It is too soon to make more than a tentative statement as to what 

 happens in artificial parthenogenesis. According to some, the arti- 

 ficial changes in the medium do not in themselves directly induce 

 segmentation, but modify the intimate constitution of the egg in 

 such a way that when it is returned to its natural medium, it 

 becomes auto-parthenogenetic. According to Loeb, the physico- 

 chemical agency induces the formation of a "fertilisation-mem- 

 brane" by a change in the surface of the egg comparable to that 

 which follows the entrance of a spermatozoon. The first step is a 

 cytolysis or partial solution of the cortical layer of the ovum, 

 perhaps a liquefaction of fatty substances in the cellular emulsion. 

 The result is the formation of the "stabilising envelope" or "ferti- 

 lisation membrane '. But the appearance of this membrane seems to 

 lead to an acceleration of the oxidations going on in the egg; the 

 egg is activated and segmentation begins. But this may simply 

 lead to disintegration, if there is not also a corrective factor, and it 

 has been possible to devise experimental conditions that induce 

 activation only and others that induce activation followed by stable 

 development. Thus the presence of a fatty acid, such as butyric, 

 may bring about membrane-formation and the activation of the 

 egg, while the presence of a hypertonic solution (i.e. with 

 increased osmotic pressure) may serve as the essential corrective. 

 The life of the activated egg may also be saved by putting it after 

 the membrane-formation for about three hours into sea-water 

 practically free from oxygen or containing a trace of potassium 

 cyanide. In either way the over-active oxidations in the egg may 

 be suppressed. If the eggs are thereafter transferred into ordinary 

 sea- water containing free oxygen, they often develop normally. 



