Dewitz, Buller, and the writer have vainly tried to prove the existence of a positive chemotropism of spermatozoa to eggs of the same species. Lillie claims to have proved a positive chemotropism of the sperm of sea urchins to “fertilizin,” but such a conclusion is only justified if a method similar to that of Pfeffer’s with capillary tubes, gives positive results; such a method was not used in Lillie’s experi­ments. It seems that the fertiliza­tion of the egg by sperm is rendered possible by two facts; first that where fertiliza­tion takes place outside the body egg and sperm are shed simultaneously by the two sexes. This can be easily observed in the case of fish. But it is also the case in invertebrates. Thus the writer has observed that the sea urchins Strongylo­centrotus purpuratus at the shore of Pacific Grove all spawn simultaneously. The examina­tion extended over several miles of shore. At such spawning seasons the sea water becomes a suspension of sperm.

The second fact guaranteeing the fertiliza­tion of the eggs is the overwhelming excess of spermatozoa over eggs. The enormous waste in animated nature is in agreement with the idea of a lack of purpose; since in this case the laws of chance must play a great rôle; and the origin of durable organisms by laws of chance is only comprehensible on the basis of an enormous wastefulness, for which evidence is not lacking.

CHAPTER V

ARTIFICIAL PARTHENOGENESIS

1. The majority of eggs cannot develop unless they are fertilized, that is to say, unless a spermato­zoön enters into the egg. The ques­tion arises: How does the spermato­zoön cause the egg to develop into a new organism? The spermato­zoön is a living organism with a complicated structure and it is impossible to explain the causa­tion of the development of the egg from the structure of the spermato­zoön. No progress was possible in this field until ways were found to replace the action of the living spermato­zoön by well-known physico­chemical agencies.[84] Various observers such as Tichomiroff, R. Hertwig, and T. H. Morgan had found that unfertilized eggs may begin to segment under certain condi­tions, but such eggs always disintegrated in their experi­ments without giving rise to larvæ. In 1899 the writer succeeded in causing the unfertilized eggs of the sea urchin Arbacia to develop into swimming larvæ, blastulæ, gastrulæ, and plutei, by treating them with hypertonic sea water of a definite osmotic pressure for about two hours. When such eggs were then put back into normal sea water many segmented and a certain percentage developed into perfectly normal larvæ, blastulæ, gastrulæ, and plutei.[85] Soon afterward this was accomplished by other methods for the unfertilized eggs of a large number of marine animals, such as starfish, molluscs, and annelids. None of these eggs can develop under normal condi­tions unless a spermato­zoön enters. These experi­ments furnished proof that the activating effect of the spermato­zoön upon the egg can be replaced by a purely physico­chemical agency.[86]

The first method used in the produc­tion of larvæ from the unfertilized eggs did not lend itself to an analysis of the activating effect of the spermato­zoön upon the egg, since nothing was known about the action of a hypertonic solu­tion, except that it withdraws water from the egg; and there was no indica­tion that the entrance of the spermato­zoön causes the egg to lose water. No further progress was possible until another method of artificial parthenogenesis was found. When a spermato­zoön enters the egg of a sea urchin or starfish or certain annelids, the surface of the egg undergoes a change which is called membrane forma­tion; and which consists in the appearance of a fine membrane around the egg, separated from the latter by a liquid (Figs. 4 and 5). O. and R. Hertwig and Herbst had observed that such a membrane could be produced in an unfertilized egg if the latter was put into chloroform or xylol, but such eggs perished at once. It was generally assumed, moreover, that the process of membrane forma­tion was of no significance in the phenomenon of fertiliza­tion, except perhaps that the fertiliza­tion membrane guarded the fertilized egg against a further invasion by sperm. However, since the fertilized egg is protected against this possibility by other means the membrane is hardly needed for such a purpose.

Fig. 4    Fig. 5
Fig. 4. Unfertilized egg surrounded byspermatozoa (whose fla­gel­lum is omitted in the drawing).
Fig. 5. The same egg after a spermato­zoönhas entered. The fer­til­iza­­tion membrane is separated from the egg by aclear space.