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Page:Popular Science Monthly Volume 86.djvu/572

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THE POPULAR SCIENCE MONTHLY

FERTILIZATION AND ARTIFICIAL PARTHENOGENESIS OF THE EGG
By Dr. J. F. McCLENDON

UNIVERSITY OF MINNESOTA MEDICAL SCHOOL

TICHOMIROFF, in 1886, was the first to use the term artificial parthenogenesis, referring to acceleration in the development of the naturally parthenogenetic eggs of the silkworm by methods found effective in hastening development in fertilized eggs of the same species. To-day the term is applied to development of eggs not usually parthenogenetic, although a few such might develop in nature under accidentally abnormal conditions.

The exact extent of development that is to be dignified by this term is a matter of dispute, some claiming it should be possible to produce an adult reproductive organism by artificial parthenogenesis. Though Delage obtained two sea urchins in this manner and more than one observer has so produced frogs, none of these reproduced a second generation, a fact not hard to understand on remembering that normally fertilized eggs of many animals have never been reared to maturity and reproductive activity under observation. Loeb considers a swimming larva to be the goal of the investigator. But it is interesting to note that the "swimming larvæ" of the marine worm Chætopterus, which he produced from unfertilized eggs, were shown by F. Lillie to be abnormal, unsegmented or poorly segmented eggs that had developed cilia.

We may consider for a moment what signifies development in the egg. The egg of any animal is in the beginning a single cell and undergoes a certain development before normal fertilization. Some animals reproduce parthenogenetically for several generations (i. e., plant lice) and the silkworm eggs, noted before, undergo more or less development if fertilization fails to occur. The eggs of most animals, however, do not segment (produce an embryo) before fertilization. Though in many of these same species (for example, sea urchin) the eggs (during maturation) undergo before fertilization two very unequal cell divisions, resulting in the formation of "polar bodies," the unfertilized egg is generally considered a single cell, since the "polar bodies" seem to have no further part in the formation of the embryo. The positive sign of development in the mature egg seems to be segmentation. We may therefore consider artificial parthenogenesis to be demonstrated by the segmentation of unfertilized eggs which do not normally segment until fertilized.

R. Hertwig was the first to observe the segmentation of unfertilized