FACTORS IN ONTOGENY 249 



various names, endowed with various particular properties, 

 and attributed, as to their origin, to varying sources. 



In the seventeenth century and early part of the eighteenth 

 century, before the time of the microscope, many naturalists 

 and physicians believed that in each germ cell (or, according to 

 some, in each egg cell, according to others, in each sperm cell) 

 there existed, preformed and almost complete, a new organism 

 in miniature, and that development was simply the expanding 

 and growing up of this tiny embryo man, or monkey, or chick. 

 Also they were forced to believe, if this first assumption were 

 true, that in each preformed embryo still smaller replicas of 

 their particular kind must exist to be the children of this child, 

 and so on, ad infinitum. Like the nests of Japanese boxes, the 

 outer one encasing a smaller and this still a smaller, and this 

 yet a smaller and so on, the young and future young of any 

 kind of organism were, according to this encasement theory 

 of the germ cell structure, nested in the egg and sperm cells of 

 any organism. 



But the invention and use of the microscope soon put this 

 theory aside. The germ cells were found to contain no pre- 

 formed embryo. Indeed, they seemed to the earlier micro- 

 scopists to be utterly homogeneous little specks or masses of 

 protoplasm, and the pendulum of speculative explanation 

 tended to swing well away from any preformation theory 

 toward the speedily formulated epigenetic theories, which 

 assumed that all germ cells were practically alike except as 

 to their paternity and maternity, and that the development of 

 these homogeneous specks of protoplasm must be determined 

 chiefly by external conditions and influences. 



However, it was obvious that there was no logical or 

 even fair reason for believing that the lack of structural 

 differentiation in the germ plasm revealed by the micro- 

 scope was a proof of the actual absence of such organiza- 

 tion. The first microscope magnified but a few hundred 

 diameters, revealing structure invisible to the unaided eye; 

 but later microscopes, magnifying objects a thousand and 

 more diameters, revealed structure and organization which 

 were quite invisible to the lower-powered instruments. And 

 so, although to-day we examine germ plasm with lenses 

 magnifying three thousand times, and yet fail to discover 

 more than threads, rods, grains, or droplets in a viscous 



