l6o BIOLOGICAL LECTURES. 



true, deduce from comparative observations on normal phenom- 

 ena, without recourse to experiment, many modi operandi which 

 obtain among the parts ; and these modi operandi will have the 

 greater probability the greater and more varied the materials of 

 observation. 



In this way Balfour (10) deduced the fact that the eggs of 

 sharks, bony fishes, and birds undergo only a partial segmenta- 

 tion from the inhibitory effects on division of the great amount 

 of yolk accumulated in the eggs of these animals. 



Nevertheless such conclusions never yield " complete " certainty 

 because the observed connection of the phenomena need not be a 

 direct one, but may depend on the effects of a third unknown 

 component, or components. For the organic processes of the 

 typical or normal development of organisms are so incompre- 

 hensibly manifold and enigmatical that, particularly in the 

 beginnings of exact catisal investigations, we can never deny 

 with assurance the existence of such a common third compo- 

 nent or other components ; and the less because in every case 

 only a small part of the secondary or tertiary phenomena fall 

 within the limits of our observation, while all the primary phe- 

 nomena of organic formation are concealed from our view. 

 Hence, modi operandi may be "ascertained" by means of com- 

 parative observations on normal phenomena, but they cannot be 

 "proved." 



This must always be borne in mind ; we can never regard 

 such effects as are concluded from mere observation of typical 

 normal phenomena as perfectly certain ; we must endeavor to 

 obtain direct proofs of these effects. 



It has been shown that in the early cleavage of many eggs 

 the directions of the division-planes follow one another in defi- 

 nite sequence. Bearing in mind that the nuclear spindle lies 

 normally at right angles to the division-plane of the cell, the 

 common result of the directions of these first divisions with ref- 

 erence to the shape of the cells in corresponding periods of time, 

 is this : in these first divisions the nuclear spindles place them- 

 selves in the longest axis which can be drawn through the cen- 

 ter of mass of the protoplasm. Starting with this statement, 

 it is possible to conclude deductively as to the sequence of the 



