CHAPTER 18 

 SOME GENERAL PROBLEMS OF DEVELOPMENT 



We are now in a position to discuss certain general problems 

 of development. All metazoa arise from eggs and not by processes 

 of spontaneous generation, as was believed when frogs and mice 

 were supposed to arise from the mud of the fields, or the human 

 embryo to be generated spontaneously in the mother's womb 

 through some mysterious influence exercised by the fluid from 

 the male. General acceptance of the Cell Doctrine and the 

 knowledge that the organism begins its life as an individual when 

 the male and female germ cells unite in fertilization estabhshed 

 the mode of origin for man as well as for other many-celled animals. 

 Development was seen to consist of the cell divisions and the cell 

 differentiations by which the diversified structures of the adult 

 are produced. Although the topics of the following discussion are 

 somewhat heterogeneous, each represents a problem that is appro- 

 priate for consideration at the conclusion of such an account of 

 development as has been given in the preceding chapter. 



Preformation and Epigenesis. — One of the most famous dis- 

 putes among the earlier embrj^ologists was that concerning pre- 

 formation versus epigenesis. Is the organism already formed 

 within the zygote, as in the bud of a plant, and does development 

 consist merely in an unfolding of what is already existent; or is 

 development the coming into being of one feature after another 

 from a beginning that is without form and void, insofar as any resem- 

 blance to the completed organism is concerned? The preforma- 

 tionists of the eighteenth century went so far as to develop an 

 elaborate theory of encasement, by which the egg was supposed 

 to contain all of the adult structures in miniature, including the 

 germs of all future generations, enclosed one after another in 

 decreasing magnitude, like toy eggs within eggs carried inward to 

 infinity. Thus, the ovary of Eve was supposed to have contained. 



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