280 Experimental Zoology 



distal end, but this is characterized by the presence of a grow- 

 ing region, which continues to add new segments to the new 

 part. 



What factor determines that the terminal organs are those that 

 are first laid down in the new part ? It cannot be the shape 

 of the new part as such, for this is practically a dome-shaped 

 knob for all new parts. The bounding surface seems certainly 

 to be a factor in this relation of the parts, as does also- the relation 

 of the old organs or layers at the cut surface. Between these two 

 boundaries the relation of the parts to each other determines the 

 result. A number of considerations, that I cannot enter into 

 more fully here, have led me to suspect that this relation of the 

 parts can be accounted for as due to a condition of stratifi- 

 cation or polarity, due to the mutual pressure of the parts on 

 each other, which acts as the stimulus for the differentiation of 

 the cells. By these same assumptions we can, I think, also give 

 a fairly consistent explanation of the difference in the rate of 

 growth at different levels. 



Let us take, by way of illustration, the results that have been 

 obtained in another worm, lumbriculus. If the worm is cut in 

 two at almost any level, there develops from the posterior end of 

 the anterior piece a new tail, and from the anterior end of the pos- 

 terior piece a new head. The material out of which these two new 

 parts develop must be identical. What determines, then, that 

 the new material forms in one case a head and in the other a tail ? 

 Since the development of these new parts seems to be largely a 

 centripetal phenomenon, we cannot assume that the influence of 

 the old part on the new, a centrifugal influence, determines the 

 result ; but since the order or sequence of the differentiation in the 

 new part is the same as that in the old part, this may determine 

 whether a head or a tail develops. In other words, the polarity 

 of the new part is in each case the same as that of the old. This 

 polarity is an expression of the stratification of the differentiation ; 

 at least, this is the most probable view of polarity, I think, that 

 we can find at present. The centripetal influence acting on 

 the new material at the anterior end determines therefore that 



