Dental review; devoted to the advancement of dentistry. . ted under a special mechanism. In this develop- ment from a single cell we will see that the individual passes throughsuccessive stages which correspond to the steps the race has passedthrough in its development, or ontogeny repeats phylogeny. The fertilized ovum is a single cell, but it contains that which de-termines the entire development and all of the functions of the adultorganism. It may therefore be compared to the single celled animaland at this stage the individual may be said to be in the aemeba con-dition. This single cell d

- Image ID: 2AM0963
Dental review; devoted to the advancement of dentistry. . ted under a special mechanism. In this develop- ment from a single cell we will see that the individual passes throughsuccessive stages which correspond to the steps the race has passedthrough in its development, or ontogeny repeats phylogeny. The fertilized ovum is a single cell, but it contains that which de-termines the entire development and all of the functions of the adultorganism. It may therefore be compared to the single celled animaland at this stage the individual may be said to be in the aemeba con-dition. This single cell d
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Image ID: 2AM0963
Dental review; devoted to the advancement of dentistry. . ted under a special mechanism. In this develop- ment from a single cell we will see that the individual passes throughsuccessive stages which correspond to the steps the race has passedthrough in its development, or ontogeny repeats phylogeny. The fertilized ovum is a single cell, but it contains that which de-termines the entire development and all of the functions of the adultorganism. It may therefore be compared to the single celled animaland at this stage the individual may be said to be in the aemeba con-dition. This single cell divides into two, then four, then eight, and soon, and these divisions take place in a definite way, constituting theprocess known as segmentation. The cell being more or less spheri-cal, the first division occurs as if by a plane passing through bothpoles dividing it into hemispheres, the second also through both polesat right angles with the first divides the hemispheres into quadrantsand the third at right angles with the other two, in a position cor-. Fig. 4. ORIGINAL COMMUNICA TIOiVS. 1129 responding to an equator divides the four cells into eight. Thisgoes on, the lines of division following regular geometrical lawsuntil the single cell is converted into a spherical mass of cells notvery much larger than the original ovum. In development, as might be expected, we find differences in dif-ferent classes of animals. While they all follow a similar develop-ment up to their degree of perfection or specialization, in one cer-tain evolutionary steps are most marked and recognizable, in othersdifferent stages are more clearly seen. In some animals as in thechick, the ovum is provided with a great amount of nutriment mate-> rial so that the active protoplasm really floats on a sphere of non-living matter. In others, as the frog, the egg contains much yolk,but only enough to make the cells around one pole after the first fewdivisions much slower in division and so much larger tha