DAVENPORT : PROCESSES CONCERNED IN ONTOGENY. 



199 



tain criteria may, however, be employed in some cases to determine 

 this. Thus, where many migratory bodies move towards a common 

 point, or where a thread or tubule makes its way to a distant point, 

 we may believe that a positively tropic stimulus is exercised by that 

 point. Also, where two similar parts move towards each other, it is 

 probable that a -\- stimulus is exerted by both ; where, on the other 

 hand, they mutually withdraw, it is probable that a mutual negative 

 stimulus emanates from both. 



With these criteria in mind we may classify some of the taxie and 

 tropic pi'ocesses as -|- or — , and this I have attempted to do in the 

 followiu"; table : — 



Probable Eespoxses to Negativk 

 Stimuli. 



7^axic. 

 I. 2, a. Migration from a layer. 

 III. 4. Thinning of a layer (when clue 

 to flowing of the protoplasm 

 from a point). 



III. 9. Perforation. 



IV. 4. Dispersal of elements. 

 IV. 5, 6. Separation of masses. 



Probable Eespox.ses to Positive 

 Stimuli. 



Taxic. 

 I. 3. Aggregation of mesenchyme. 

 I. 4. Attachment of mesenchyme. 

 I. 5. Investment and interpenetra- 



tion. 

 I. 6. Transportation. 



III. 4. Thickening of a layer (when 



due to flowing towards one 

 point). 



IV. 7. Fusion of masses. 



Tropic. 

 II. 1. Turning of thread. 

 II. 3. Anastomosing. 

 II. 4. Union with other organs. 

 III. 8. Concrescence. 



Probable Responses to either -|- or — Stimuli. 



I. 1. Migration of nodal thickenings. 

 I. 2, b. Free migration nf amoeboid bodies. 

 IV. 2. lloarrangement of nuclei. 



It is not too much to believe that the foregoing hypothetical interpre- 

 tation of the ontogenetic processes lies within the po.ssibility of experi- 

 mental test. Just as the control of the migration of amoeboid bodies in 

 the adult has been undertaken wMth success, so may we hope to control 

 the tropic and aggregation phenomena of ontogeny. By ex2)eriment alone 

 can the causes of the developmental processes be determined. 



Cambridge, Mass., May 1, 1895. 



