FERTILISATION 233 



treatment with hypertonic sea- water in the way described. The 

 substances which called forth membrane formation can be divided 

 into two groups, the first consisting of hydrocarbons and certain 

 substitute products, and the second being comprised of certain acids. 

 Loeb states also that the order in which the two agencies are employed 

 is of vital consequence, for if th'e eggs are subjected to the membrane- 

 forming solution after being placed' in the hypertonic sea- water 

 instead of -before, they develop a membrane, but shortly afterwards 

 disintegrate. As a result of this series of experiments he concludes 

 that the process of membrane formation is an essential and not a 

 secondary phenoinenon.^ He makes the further suggestion that 

 membrane formation is brought about by a kind of secretory process, 

 due to the squeezing out under pressure of a liquid substance from 

 the interior of the ovum ^ (c/. Jenkinson, p. 182, above). According to 

 this view the excretion of the fluid is the essential feature, while the 

 actual formation of the membrane is probably only a secondary 

 mechanical effect of the sudden secretion. 



In the case of the starfish it was found that the process of artificial 

 membrane formation was alone sufficient to induce parthenogenetic 

 development without any further treatment with hypertonic sea-water. 

 This observation is connected by Loeb with the fact that starfish 

 eggs are sometimes able to develop in the absence of any external 

 cause or agency. Parthenogenetic development of starfish eggs has 

 been produced also by mechanical agitation ; ^ but it is possible, as 

 Loeb observes, that the diffusion of carbonic dioxide, or some other 

 gas, into or from the eggs may be the real exciting cause.* 



Loeb - found also that the unfertilised eggs of, the Annelid 

 Chcetopterus could be made to develop- into swimming larvae by 

 adding a small quantity of a soluble potassium salt to the sea- water 

 in which they were placed.* The same result could be brought about 

 by the addition of hydrochloric 'acid. The eggs appeared to undergo 

 development, as far as the trochophore stage, but without segmenting. 



Lillie^ (r.),however,found that the nuclear divisions were abnormal, 

 and that the apparent trochophore larvae were not typical, being in 

 reality merely " ciliated structures " which were far behind the real 



1 It was found, however, that in the case of the starfish a very small number 

 of eggs could, develop without first forming a niembrane, and that this 

 number could be increased by transitorily treating the eggs with acidulated 

 sea- water. See below. 



2 Loeb, "Ueber die Natur der Losungen, etc.," Ffliiger's Arch., vol. ciii., 1904. 

 ^ Mathews, "Artificial Parthenogenesis produced by Mechanical Agitation," 



Amer. Jour, of Phynol., vol. vi., 1901. 



'' Loeb, The Dynamics of Lwitig Matter^ New York, 1906. 



^ Loeb, " Experiments on Artificial Parthenogenesis in Annelids, etc.," Amer. 

 Jour, of Physiol., Yo\. iv., 1901. 



^ Lillie, "Differentiation without Cleavage in the Egg of the Annelid 

 Chcetopterus pp-ffamentacetis," Arch. f. Entwick.-Meohanil; vol. xiv., 1902. 



8 A 



