^^JO HOLMES. [Vol. XVI. 



The investigation, the results of which are recorded in the 

 present paper, was carried on under the supervision of Prof. 

 C. O. Whitman, at the University of Chicago and at the Marine 

 Biological Laboratory at Woods HoU, Mass. It is a source of 

 gratification to acknowledge the generous treatment I have 

 received in both these places at Professor Whitman's hands. 

 I wish also to express my appreciation of the many suggestions 

 I have received from Dr. E. G. Conklin, both in person and 

 through his admirable paper on the " Embryology of Crepidula." 



The species studied is Planorbis trhwlvis Say. This species 

 occurs in great abundance in the ponds of South Park, Chicago, 

 and it was also found in a pond near Falmouth, a few miles 

 from Woods Holl. 



Part I. Descriptive Portion. 

 Methods. 



The eggs of Planorbis, when brought into contact with water, 

 swell quite rapidly, so it is best to tease them out of the cap- 

 sules directly into fixing fluid. The eggs may, however, be 

 teased first into normal salt solution, to which it is better to 

 add a small quantity of the fixative, and then treated with the 

 fixing fluid alone. Some fixing fluids coagulate the albuminous 

 substance around the ^g^ so quickly, after or during its escape 

 from the capsule, that it usually becomes surrounded with more 

 or less coagulated albumen, from which it is difficult to free it. 

 By teasing directly into normal salt solution, mixed with only 

 a small quantity of the fixative, the eggs may be obtained free 

 from any foreign material. Normal salt alone often causes more 

 or less swelling. 



Formalin does not appear to coagulate the albuminous sub- 

 stances in the capsules. The &g^^ masses may be kept in a 5^ 

 solution of this substance for several days, the jelly remaining 

 perfectly fluid and transparent. Unfortunately, formalin does 

 not otherwise prove a satisfactory fixing agent. Kleinenberg's 

 stronger picro-sulphuric gave good results, especially when 

 followed by the method of staining with acidified Delafield's 



