Heredity / by J. Arthur Thomson.

  • Thomson, J. Arthur (John Arthur), Sir, 1861-1933.
Date:
1908
Catalogue details

Licence: In copyright

Credit: Heredity / by J. Arthur Thomson. Source: Wellcome Collection.

Provider: This material has been provided by King’s College London. The original may be consulted at King’s College London.

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    normal number of eighteen chromosomes : such are some of the extraordinary results reached by this clever experimenter. It seems, then, as if fertilisation may, in many cases, be effective without there being any ovum-nucleus present, as if the essential fact were the union of a sperm with a mass of egg- cytoplasm. Delage's experiments cited above seem to prove that the nucleus and centrosome of the ovum are not essential to ferti- lisation. Professor Loeb (1899), of Chicago, has made experi- ments which seem to show that the spermatozoon may be dispensed with. In other words, he has been able to induce parthenogenetic development artificially in cases where it does not normally occur. He has been led to believe that the only reason why the eggs of many marine animals do not develop parthenogenetically is that something in the constitution of the sea-water prevents it. This something is the presence or absence of ions of sodium, calcium, potassium, and magnesium, the two former requiring to be reduced, the two latter to be increased. The mixture of about 50 per cent, ^^n MgCla (magnesium chloride) with about 50 per cent, of sea-water was able to bring about the same effect as the entrance of a sperma- tozoon. The unfertilised eggs [of the sea-urchin Arbacia] were left in such a solution for about two hours. When brought back into normal sea-water they began to segment and form blastulae, gastrulae, and plutei, which were normal in every respect. The only difference was that fewer eggs developed, and that their development was slower than in the case of the normal develop- ment of fertilised eggs. With each experiment a. series of control experiments was made to guard against the possible presence of spermatozoa in the sea-water. . . . From these experiments it follows that the unfertilised egg of the sea-urchin contains all the essential elements for the production of a perfect pluteus. The only reason that prevents the sea-urchin from developing par- thenogenetically under normal conditions is the constitution of