292 THE AMERICAN MONTHLY [Sept. 



The object of this paper is simply to show the develop- 

 ment of one of the free- swimming Campanularian meduste 

 — that of Obelia commissuralis, while no attempt is made 

 to describe the sexual method by which the medusa3 give 

 rise to the hydroidal forms. 



This particular specie is found growing along the rocky 

 shores of the Atlantic Ocean, from Nova Scotia to Charles- 

 town, South Carolina, attached to stones or sea-weeds of 

 various sorts. The material for this work was found 

 growing on the ro})es attached to lobster pots which were 

 set near the Biological Laboratory, Cold Spring Harbor, 

 Long Island. On these ropes the hydroids were found 

 growing luxuriantly, even to a considerable depth below 

 the surface of the water. The material having been 

 collected, four different fixing solutions were made use of 

 in preserving it. Corrosive Sublimate, Perenye's Fluid, 

 Fleming's Solution, and Picro-sulphuric Acid. The 

 latter proved the most satisfactory, preserving the tissues 

 so as to show the cellular structure very distinctly. 

 The material having been treated with tliese various 

 fixing solutions, was than preserved in alcohol, and later 

 the development was made out by staining and cutting 

 sections according to the usual method. 



The reproductive organ of Obelia consists of a repro- 

 ductive calycle (fig. 1, a) which occupies the forks of 

 branches and is composed of a horny sheath (fig. 2, b) 

 which surrounds a central portion, the manubrium. The 

 manubrium, in accordance with the general structure of 

 the Coelenterates, is composed of two cellular layers, the 

 ectoderm and the endoderm and on this manubrium the 

 the medusse are developed by a process of budding. The 

 first step to be noticed in the development is a slight thick- 

 ening of the ectodermal layer of cells (fig. 3, c) on one 

 side of the manubrium of the calycle. Soon, however, both 

 ectoderm and endoderm push out from the axis of the man- 



