yS MARINE BIOLOGICAL LABORATORY. 



hypothesis, and this solution is one that many embry- 

 ologists would be unwilling to accept. I think the great 

 majority of morphologists will agree that no satisfactory 

 explanation of metamerism has yet been given ; and the 

 problem of apical growth is still farther from a solution. 

 Concrescence stands on a very different footing, since 

 clear and definite causes for it can be assigned ; yet 

 even here a complete solution of the problem will only 

 be possible when comparative embryology has advanced 

 far beyond its present standpoint. As regards the 

 trochophore, opinion is still divided ; and I am giving 

 only a personal view in stating that the accumulating 

 evidence seems to favor, in the main, the view that it 

 is a secondary larval form, which gives no clue to the 

 ancestory of the segmented animals. To those whose 

 interest in science lies in the consideration of its posi- 

 tive results only, the outcome of this discussion will 

 doubtless seem rather unsatisfactory ; and it must be 

 admitted that in some respects the fundamental prob- 

 lems of annelid and vertebrate morphology seem to be 

 as far from a solution as in the time of von Baer. To 

 the investigator, however, it is the unsolved problems 

 that call forth the deepest interest. It is the very 

 vagueness and uncertainty of the subject that impress 

 upon us how much remains to be done in the embry- 

 ology of annelids, and arouse the interest with which 

 we look forward to the results of future investigation in 

 this field of study. That the problems of metamerism 

 and apical growth will ultimately be solved, there can 

 be little doubt ; but the present need is for new facts, 

 not for new theories. When the facts are forthcoming, 

 the theories will take care of themselves. 



