l8o BIOLOGICAL LECTURES. 



and to these in part in constant directions, are influenced even 

 in their typical morpJiology to a great extent by ''external'' 

 factors, zuhereas the "typical'' structure of animals, which are 

 capable of active locomotion, is in great measure independent of 

 external formative influences and consists, apart from certain 

 functions of superficial parts, in self-differentiation. It is, how- 

 ever, much more difficult to understand the internal than the 

 external factors and the reactions to the same. 



In sessile animals J. Loeb (24) has recently discovered dif- 

 ferentiating effects of gravity on the organism, like those 

 observed in plants. For example an inverted piece of a hy- 

 droid polyp will produce roots at its lozver and shoots at its 

 upper end. But we must be careful not to extend this occur- 

 rence to other animals, as has already been done, thus ignoring 

 the causal implication in the sessile mode of life, and ascribing 

 in all animals a differentiating effect to gravity, especially 

 when irreproachable experiments have already proved the 

 opposite in the case of other animals. 



Of particularly great importance to developmental mechan- 

 ics, are, furthermore, many of the results of the pathological 



SCIENCES. 



Lookino- aside from the cases in which immediate death is 

 brought about by a sudden stopping or disturbance of the 

 functions which are necessary to keep the machine going, we 

 observe in every primary disturbance, no matter how it may be 

 caused, secondary changes intervening, which even though they 

 be merely fimctional at first, nevertheless gradually lead to 

 formative changes. 



In this manner these secondary formative changes give us evi- 

 dence of formative interrelations, formative modi operandi of 

 parts one upon another, an understanding of zvhich is essential to 

 our purpose. 



But even here, as in the effect of an experiment, we must 

 first , ascertain whether these pathologically formative modi 

 operandi enable us to draw any conclusions whatever with 

 respect to normal operations, or whether under abnormal con- 

 ditions abnormal modes of reaction may also occur, and hence 

 processes which do not occur at all in normal phenomena. 



