7O A. RICHARDS. 



found to differ much. Reference to these more recent observa- 

 tions will be made in the appropriate places under the later dis- 

 cussion. 



METHODS. 



Specimens of Planorbis may be kept in aquaria, and during 

 the night they will lay on the sides of the glass dishes or on the 

 water plants that may be in the aquaria. If lily pads are placed 

 in the aquaria, their rough lower surfaces seem to be favorable 

 places for finding clusters of these eggs. The eggs occur in 

 "clusters" (Holmes) which are bound together by tough enclos- 

 ing membranes and which contain a considerable amount of 

 jelly. Within the clusters there are, perhaps, a couple of dozen 

 capsules filled with yellow albumen mass in which the Planorbis 

 egg itself develops. 1 



The eggs of Planorbis are mostly laid at night, or usually just 

 before day. Observations made early in the morning, say 7 



FIG. 1. Egg cluster of Planorbis, showing eggs within the albumen capsules, 

 all of which are surrounded by a gelatinous mass. (From Holmes.) 



A.M., frequently find eggs which have not yet put out the first 

 polar body. 



In studying the eggs of Planorbis, the general method of pro- 

 cedure has been as follows : the clusters of eggs have been removed 

 from the sides of the glass dish or from the plants upon which they 

 were deposited by inserting a sharp knife between them and the 

 attachment, care being taken not to break the gelatinous capsules. 

 The eggs can now be studied under conditions which do not differ 



1 See Holmes's description, Jour. Morph., Vol. 16. 



