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 



