File:The American Museum journal (c1900-(1918)) (17538946863).jpg

Title: The American Museum journal
Identifier: americanmuseumjo06amer (find matches)
Year: c1900-(1918) (c190s)
Authors: American Museum of Natural History
Subjects: Natural history
Publisher: New York : American Museum of Natural History
Contributing Library: American Museum of Natural History Library
Digitizing Sponsor: Biodiversity Heritage Library

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THE DEVELOPMENT OF A MOLLUSK.' A GUIDE TO THE SERIES OF MODELS ILLUSTRATING THE DEVELOP- MENT OF CREPIDULA. By B. E. Dahlgren, D.M.D., American Museum of Natural History. Introduction. HE problem of how living organisms arise must have ever presented itself to the questioning mind. The processes involved in the origin of new individuals nevertheless remained for ages an unsolved mystery. The most familiar ex- ample, the origin of the young bird from an egg, cannot have failed to arouse the interest even of primitive man. It must also have furnished the first suggestion towards an explanation. Although undoubtedly long unsuspected, in time it became known that every animal which does not multiply by simple division into two like the very lowest arises from an egg, which is either hatched or developed within the body of the parent. Until a century and a half ago it was generally believed that the egg contained a miniature animal, which became perfected during incubation. Not until the substance called proto- plasm had been recognized as the universal "physical basis of life," and, by the aid of the microscope, all living bodies had been found to be composed of cells, was anything like a correct understanding of the nature of the egg and its development attained. The egg was found to be a cell derived like all other cells by the division of a preexisting cell. Its development, resulting in the formation of the myriad cells of a new individual, was foimd to proceed by a process of cell-division, essentially similar to that by which growth takes place in the adult. ' Issued also in separate form as Guide Leaflet No. 21. 29

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