a film, which formed on the surface of the solution, sometimes on the 

 second day, generally during the first week, and occasionally not until 

 the nineteenth day from the commencement of the experiment. The 

 solutions consisted of mixtures of sugar and starch, with some albu- 

 minous matter, of the juice of beef sometimes filtered, and in other 

 instances containing muscular fibre and vegetable substances. Five 

 experiments were tried in flasks hermetically sealed at the begin- 

 ning, and then immersed in boiling water, in all of which Infusoria 

 were formed. 



Four experiments were made with sealed flasks in a Papin's diges- 

 ter, two of them under a pressure of two, and two under five atmos- 

 pheres. Infusoria were found in one of each. 



The organisms consisted of Vibrio, Bacterium, Torula, minute 

 AlgsB, also of small, round or oval bodies, moving with vibrating 

 cilia. 



Dr. B. J. Jeffries presented his Annual Report as Curator 

 of tlie department of Microscopy, which was accepted. 



June 4, 1862. 

 The President in the chair. 

 The following communication was presented : — 



On the Mode of Development of the Maeginal Tentacles 

 OF the Free Medusa op some Hydroids. By A. Agassiz. 



The uniformity of the order of appearance of the partitions of 

 Polyps, which was first pointed out by Milne-Edwards and Haime,* 

 led me to suppose that something similar would be the case for the 

 order of succession of the marginal tentacles of Hydroid Medusas. 

 As we have not in Acalephs new chambers, formed by additional 

 tentacles, as is the case in Polyps, I have not used the term cycle 

 of Milne-Edwards and Haime, but have employed instead of it the 

 words set of tentacles, to denote those tentacles which are formed at 

 the same time. For instance, a young Medusa just escaped from the 

 reproductive calycle has four tentacles, — they are the tentacles of the 

 first set ; the tentacles which next make their appearance, — also four 

 in number, — in the intermediate spaces, are those of the second set ; 



* Kecherches sur la structure et la classification des Polypiers r^cents et fos- 

 siles, par MM. IT. Milne-Edwards et Jules Haime, extrait des Annales des 

 Sciences Naturelles, Paris, 1848-49. 



