Pr. Henri Blanc on Ceratium hirundinella. 445 



lakes, and communicate some new and exact observations 

 upon the mode of reproduction of this group of microscopic 

 creatures, which has hitherto been very problematical. 



The bodj of the Ceratium of the Lake of Geneva, seen in 

 front, is flattened, seen from the side nearly biconvex. It is 

 surrounded and limited by a skeletogenous membrane or 

 cuirass, which is produced in the shape of horns differing in 

 dimensions and directions. 



One of these horns (fig. 4, a), the largest, is denominated 

 the anterior horn, the animal always moving in the direction 

 of this extremity ; the three others (fig. 4, 6), or sometimes 

 only two, of unequal length, are the posterior horns. This 

 kind of external skeleton is not continuous throughout, and is 

 not everywhere of the same thickness. About the middle of 

 the body it becomes excessively delicate and forms a perfectly 

 well-marked cincture (fig. 4, c) ; and the skeleton is also 

 completely interrupted upon one of the surfaces of the body. 

 This interruption is formed by a large groove with parallel 

 margins, nearly perpendicular to the cincture (fig. 4, d) ,• it 

 opens near the base of what I will for the moment call the 

 great posterior horn, and terminates in a rounded end at the 

 level of the cincture or a little below it. The width of this 

 groove, measured in several specimens, varies from 0'014 to 

 0"01o millim., its length is O^OSO millim. It is the surface 

 of the body which presents this groove that I call the ventral 

 surface, the other, which has nothing of the kind, being the 

 dorsal surface. 



The skeletogenous membrane is not of a siliceous nature, 

 as Prof. Brun says, but is formed, as M. Bergh perceived, 

 of cellulose or of a very similar carbon hydrate ; for in Ceratia 

 treated with iodized chloride of zinc, the skeletogenous mem- 

 brane always became slightly tinged with violet. This 

 cuirass is very transparent, presents no pores, and possesses 

 a very elegant structure. Under a low power it appears to 

 be very regularly divided into a quantity of little polygonal 

 dark areas, separated from each other by small white bands ; 

 under a high power we seem to see that these polygonal areas 

 are so many little fillets [Leisten) which, towards the ex- 

 tremity of the horns, become elongated, and by their super- 

 position cause the contours of the latter to appear as if slightly 

 denticulated. 



The skeletogenous membrane encloses a protoplasmic mass 

 which contains a nucleus. The protoplasm, which penetrates 

 to the extremity of the horns, is differentiated into a very thin 

 external part, the exoplasm (fig. 5, ex)^ and a larger internal 

 part, the endoplasm (tig. 5, en). 



