HABITS, ETO.j OF CEREBRATULUS LACTETJS. 101 



ducts capable of producing normal embryos. A clean glass 

 vessel (4 to 6 gals.) with plenty of the sand or mud from 

 which the animals were taken^ and a constant but slow 

 renewal of the water, has proved to be the best aquarium. 

 Only one pair of worms should be placed in a single vessel, 

 and the supply of plant life should be very meagre. Under 

 these conditions no difficulty has been experienced in keeping 

 the worms for six or eight weeks, and they have laid pro- 

 fusely (cf. Coe, 15). 



2. The ease and uniformity with which the egg may be 

 fertilised ai-tificially. 



3. The persistence and remarkable activity of the 

 polar bodies. They remain in close connection with the egg 

 all through segmentation and gastrulation, the second body 

 being actually connected with the blastomeres by protoplas- 

 mic processes, while both display to a marked degree those 

 protoplasmic activities which have been designated as 

 " spinning." 



4. The transparency of the larva subsequent to gastru- 

 lation, which renders it very easy to follow its internal 

 development. It is so clear that very high powers may be 

 successfully used. An excellent opportunity is thus afforded 

 for the study of muscular development, owing to the peculiar 

 nature of the mesenchyme cells. 



Every fact of importance has been included in the present 

 paper ; when not verified by personal observation, due ac- 

 knowledgment has been made. The author desires to 

 express his indebtedness to Librarian Wm. I. Fletcher, of 

 Amherst College, for the free use of the science library of 

 that institution, without the help of which the present work 

 would have been impossible. It is likewise a pleasure to 

 acknowledge the inspiration and the many valuable sugges- 

 tions given by Dr. E. A. Andrews, of Johns Hopkins Uni- 

 versity, and by Mrs. G-. F. Andrews. And to Dr. W. E. Coe, 

 of the Sheffield Biological Laboratory, acknowledgment is 

 due for the kindly loan of mounted material illustrative ot' 

 spermatogenesis, and for much friendly criticism. 



