198 BIOLOGICAL LECTURES. 



and of the new tail is determined by the obliquity of the exposed 

 part. The nearer the cut approaches a plane at right angles to 

 the long axis, the nearer the new head and tail will be in the 

 middle line. 



If we assume that specific substances bring about the develop- 

 ment of the new head and tail in these pieces, and also assume 

 that the one substance flows towards the most anterior end of 

 the piece, and the other towards the most posterior end, we 

 should have an apparent explanation of the results. 



A further application of this hypothesis may be made to 

 certain phenomena in the regeneration of the earthworm. If 

 the worm (Allolobophora fcetida) is cut posterior to the middle 

 into two pieces, the posterior piece produces in many cases, at 

 its anterior end, a new fail and not a head. If we assume that 

 in the tail region only tail-forming substances are present, and 

 little or no head-forming substance, we might offer this suppo- 

 sition as an explanation of the result. But a difficulty arises 

 in connection with the idea of transportation of this stuff, for 

 on the hypothesis it should flow backwards instead of forwards. 

 However, the unusually long time before this sort of regener- 

 ation begins might be utilized to save the transportation theory. 

 We might assume that the tail-forming material flowed posteri- 

 orly, but after a time so much will have accumulated that it may 

 extend forward even to the anterior end, and there start the 

 regeneration. It is obvious that this is a forced interpretation 

 and that the result is not in accordance with the transporta- 

 tion idea. 



This example of heteromorphosis in the earthworm is due, it 

 appears, to influences within the piece itself. This seems true 

 also for those cases, described by Herbst and myself, in which 

 an antenna is regenerated in place of an eye in some of the crus- 

 taceans. This case also does not harmonize well with the trans- 

 portation hypothesis, although, as I shall try to show later, it 

 might be explained on the assumption of specific materials in 

 the parts themselves. 



There are other cases of heteromorphosis in which, as shown 

 by Loeb, the influence that determines the kind of regener- 

 ation comes from the outside. In certain hydroids regener- 



