184 BIOLOGICAL LECTURES. 



from pressure (in reality only apparently thus protected) like 

 the arachnoidea, dura-mater, in the atrophied eyeball. 



The property of self -maintenance or self -differentiation of parts 

 is evinced by the development of very minute detached por- 

 tions of tumors which may be carried anywhere by the blood 

 current and grow to be secondary tumors of the same morpho- 

 logical character as the primary tumor ; the development of 

 sporadic masses of gray brain-substance ; the retention of the 

 normal structure in abstricted pieces of the retina lying out- 

 side the eye ; the formation of hair and teeth in dermoid 

 cysts ; the teratomata ; the healing over of transplanted skin, 

 bones, eyeballs, etc. 



To these examples of the important developmental mechanical 

 results of pathological research should be added further those 

 cases of aberrations from the normal which accrue from a 

 study of monsters, and the lesser deviations designated as 

 varieties. 



Besides the varieties which may fall under the observation 

 of anatomists, there are a great number of these "experiments 

 of nature" to which especially pathological anatomists and clini- 

 cians have access. 



It would, therefore, be most serviceable and advantageous 

 to developmental mechanics if those investigators to whom 

 such phenomena present themselves were more mindful than 

 they have been heretofore of the importance of these facts in 

 ascertaining normal formative causes, and if they would for this 

 reason endeavor to collect all the formative modi operandi of 

 -which there is evidence, together with more accurate data concern- 

 ing their magnitude and time relations, their mode of operating, 

 their connections, and remoter causes. 



It is probably best to begin with an attempt to formulate 

 concisely every such phenomenon as a modus operandi. Such 

 an attempt shows at once the unsatisfactory condition of our 

 present knowledge, and there follows as a matter of course the 

 necessity of rendering this knowledge more complete. 



The same purpose would be served by many observations 

 which pathologists might make during experimentation under- 

 taken with other aims in view. Thus, e.g., in experiments on 



