100 CHAS. B. WILSON. 



In tlie present investigations for the first time^ so far as 

 can be determined, the eggs of a Nemertean were fertilised 

 artificially. 



The consequent abundance of embryos has rendered it 

 possible to follow in detail every step in their development, 

 and to use living material far more than has ever been done 

 heretofore. 



In much of the recent embryological work there has been 

 a tendency to confine the attention almost exclusively to the 

 study of sections and prepared material. And embryos have 

 been reared for the sole purpose of furnishing such material. 

 But it is clearly evident that the true purpose of histology 

 is to supplement, and not to supplant, the study of the 

 living animal. For this reason the morphological drawings 

 in the present paper have been made, whenever it was 

 possible, from living specimens, with the aid of the camera 

 lucida. 



Adults have also been captured and kept in aquaria where 

 their habits could be closely watched, and many interesting 

 facts have been thus discovered. Eggs have been laid and 

 fertilised by individuals thus kept in confinement, so that it 

 has been possible to verify all the details obtained from the 

 artificially fertilised eggs, and, in addition, to discover the 

 method of egg-laying and fertilisation. 



The development and habits of the pilidium have been 

 carefully watched for four successive summers, and as a 

 result it is possible to present quite a full account of this 

 particular species. A second result has been the growing 

 conclusion that this species is particularly fitted for embryo- 

 logical work in our laboratories. Many different reasons 

 have led to this conclusion, among which may be mentioned : 



1. The vitality of the adults, by reason of which uninjured 

 sexual products can be obtained from them even under very 

 adverse circumstances. When reduced to such a dire neces- 

 sity, a single piece of a disruptured worm, but a few inches 

 in length, can be used at the rate of half an inch a day for a 

 week or more, and it will yield all the while germinal pro- 



