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 processes as — or —, and this I have attempted to do in the 
following table: — 
PROBABLE Responses TO POSITIVE | PROBABLE RESPONSES TO NEGATIVE 
STIMULI. STIMULI. 
Taxic. T'axic. 
I. 8. Aggregation of mesenchyme, I. 2,a. Migration from a layer. 
I. 4. Attachment of mesenchyme. III. 4. Thinning of a layer (when due 
I. 5. Investment and interpenetra- to flowing of the protoplasm 
tion. from a point). 
I. 6. Transportation. III. 9. Perforation. 
III. 4. Thickening of a layer (when | IV. 4. Dispersal of elements, 
due to flowing towards one | IV. 5, 6. Separation of masses. 
point). 
. 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. 
1.1. Migration of nodal thickenings. 
I. 2,0. Free migration of amoeboid bodies. 
1V.2. Rearrangement of nuclei. 
It is not too much to believe that the foregoing hypothetical interpre- 
tation of the ontogenetic processes lies within the possibility of experi- 
mental test. Just as the control of the migration of amoboid bodies in 
the adult has been undertaken with success, so may we hope to control 
the tropic and aggregation phenomena of ontogeny. By experiment alone 
can the causes of the developmental processes be determined. 
CAMBRIDGE, Mass., May 1, 1895. 
