THE CELL AS THE ELEMENTAL GERM OF AX ORGANISM 335 



another more closely than do the egg- and sperm-cells of the same 

 animal. 



However, these similarities and differences in form appear to be 

 of less importance when we go more deeply into the subject. For, 

 as men, rodents, ruminants, and invertebrate animals present to 

 us more or less important external differences, the sexual cells 

 originating from them must differ in a corresponding manner as 

 regards their fundamental attributes, in so far as they represent 

 the embryonic stage of the subsequent complete organism. The 

 only thing is that, at present, the essential differences lie beyond 

 our perception. On the other hand the egg- and sperm-cells of 

 the same organism, although they differ so much in external 

 appearance, must resemble one onother in their essential properties, 

 since they must contain potentially all the characteristics of the 

 fully-developed animal. 



Nageli pertinently remarks (IX. 26) : " The egg cells must 

 contain all the essential characteristics of the mature organism, 

 and hence they must differ as much from one another, when they 

 are in this early stage, as when they are more fully developed. 

 The Hen's egg must possess the characteristics of its species as 

 completely as the Hen, and hence must differ as much from the 

 Frog's egg as the Hen does from the Frog"." 



What is true of the egg is equally true of individual cells and 

 collections of cells, which, being detached from the mother or- 

 ganism, either as spores or buds, are able to reproduce the parent. 

 They, too, must possess all the essential properties of the whole, 

 in an embryonic condition, although they are imperceptible to us. 



What idea can we form to ourselves of these invisible properties 

 of the cells, which predetermine the complex organism ? What is 

 the connection between the developed and undeveloped stage ? 



These problems are amongst the most difficult which the theory 

 of life presents. Scientists and philosophers have occupied them- 

 selves with these questions for centuries, and have formulated 

 their conclusions in hypotheses, which have frequently influenced 

 enquiry. We will mention shortly those theories which are most 

 important historically, since they are both of general interest, 

 and will serve as a suitable introduction to the consideration of 

 the views, which are suggested by modern research. 



I. History of the older Theories of Development. Two 



important scientific theories which are directly opposed to one 



