30 THE AMERICAN MUSEUM JOURNAL 



Out of the discovery of the character of the egg, of its origin 

 from a parent cell and of its processes of development grew 

 numerous other problems demanding the attention of investi- 

 gators. Thus the science of embryology came into existence. 

 This science seeks to discover every step in the development of 

 an organism and to trace resemblances and differences of structure 

 and form from their very earliest beginnings. It investigates 

 the conditions which influence development and seeks to dis- 

 cover the factors which determine each step in the formation of 

 an organism, to what extent development is dependent upon 

 external causes and to what extent it is predetermined by the 

 internal organization of the egg. It seeks to determine pre- 

 cisely what this internal organization is and to explain the manner 

 in which the reproductive cell becomes the bearer of the charac- 

 ters of the parents and by what process it is able to transmit 

 these to the offspring. 



The comparison of the development of different animals soon 

 revealed striking similarities at certain stages. It was found 

 that after cell-division had proceeded to a certain extent the de- 

 veloping egg assumed a form resembling a mulberry (the morula) ; 

 that later the cells invariably became arranged in the form of a 

 hollow sphere (the blastula) , this in turn giving rise to a some- 

 what more complicated flask-shaped form (the gastrula) . It was 

 seen that these various stages presented remarkable correspond- 

 ences to certain lower forms of life. The analogy of the un- 

 divided egg to a simple unicellular protozoan ; of the mulberry, 

 or morula, stage to simple aggregations of unicellular animals 

 such as are found among the lowest forms of life ; of the blas- 

 tula to certain Flagellates which occur in the form of hollow, 

 free-swimming, multicellular spheres, and the apparent analogy 

 of the gastrula to certain polyps led to the theory that the 

 developing animal, in the course of its formation from the egg, 

 passes successively through the forms of a whole series of lower 

 organisms which may be considered as its ancestral types. 



Formulated at a time when the evolution theory had been 

 recently advanced, this corroborative theory aroused the liveliest 

 interest. Although the original theory has been largely modi- 

 fied since the developmental history of a greater number of 



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