DEVELOPMENTAL MECHANICS. l6l 



first planes of cleavage. At the same time, this statement is 

 not certain so long as it cannot be proved directly ; for the 

 same typical sequences of division in normal development might 

 be brought about by other, albeit, perhaps, 7mich viore compli- 

 cated, but, nevertheless, typical operations. More than by a 

 hundred further agreements with the rule in normal phe- 

 nomena, the approximate truth of the above statement was 

 proved by a single experiment, in which by pressing the eggs 

 till they assumed an abnormal form, the sequence of the planes 

 during the early cleavage departed from the normal, but even 

 in this condition the nuclear spindle came to lie in the above- 

 mentioned greater axis. At the same time it was shown, how- 

 ever, that in rare cases the nuclear spindle places itself in the 

 smallest axis which can be drawn through the center of mass of 

 the protoplasm, a fact which points at the same time to the 

 operation of several factors in the determination of this direct- 

 ive influence (5). 



Among biologists there is a tendency derived from the inor- 

 ganic sciences, to regard the Jiypothetical deductiotis which ap- 

 pear to us to be the ''simplest,'' as having the greatest proba- 

 bility for the very reason that they seem so simple. 



Although much has been done on this assumption, and 

 unfortunately must be done, and although much that is true has 

 already been brought to light, nevertheless this method must 

 always be applied with great reserve to normal biological phe- 

 nomena, for deeper knowledge shows us that zve have not yet a 

 sufficient insight into the actual mechajiisms of development to 

 venture an opinion as to what may be easiest and simplest for 

 these same ^nechanisms. 



Thus we suppose that we are really simplifying matters when, 

 e.g., we attribute in consequence of functional adaptation many 

 typical and purposive forms to the self-constructive effects of 

 use. The correctness of this principle and of its application in 

 many cases has long been capable of direct proof. Neverthe- 

 less, we observe that many structures which might be the result 

 of this principle, e.g., the form of joints, the functional struc- 

 ture of the gut, are already established before there is an oppor- 

 tunity for them to exercise their definitive functions. Hence 



