450 Dr. Henri Blanc on Ceratium hirundinella. 



skeletogenous membrane. The ventral aperture is no longer 

 the same, but is only a very narrow groove ; from the base of 

 the right posterior horn (the Ceratium being seen from its 

 ventral surface) starts a groove, passing (and this is an 

 important point to ascertain) exactly over the constricted 

 portion of the nucleus. This groove traverses the circlet to 

 reach the left-hand margin of the posterior [anterior] region of 

 the body, and is continued upon the dorsal surface ; it conse- 

 quently divides the individual into two dissimilar parts, but 

 each containing half of the nucleus. 



If the reader has carefully followed me in the description 

 that I have just given of the three individuals drawn in 

 figs. 6, 7, and 8, he will have no difficulty in recognizing 

 with me that I have had before me three important stages of 

 the Ceratium under consideration, and that they enable us to 

 conclude that its reproduction takes place by division. 



Thus, to sum up, this reproduction commences by the 

 division into two equal parts of the original single nucleolus 

 contained in every nucleus. (According to Prof. Gruber* 

 the division of the nuclei of Amceha proteus also commences 

 by the division of the nucleoli.) The two halves of the 

 nucleolus separate from each other and the nucleus acquires 

 a regular oval form, although its contents do not differ from 

 what they were before. Then the nucleus becomes con- 

 stricted in the middle, and this constriction becomes more 

 and more strongly marked until the nucleus appears to be 

 formed of two halves united by a sort of bridge of nuclear 

 substance. At the same time that this constriction becomes 

 more marked the nucleus changes its position ; one of its 

 halves is situated above and the other below the cincture. 

 At this period the skeletogenous membrane presents a furrow 

 which divides it, and causes the Ceratium to appear as if 

 incompletely divided into two. The scissiparity, therefore, is 

 not truly longitudinal, but still less transverse. 



M. Bergh t says that he has frequently had the oppor- 

 tunity of observing Ceratia, especially Ceratium cornutiunj 

 which were deprived of a portion of their skeleton ; I have 

 also had the pleasure of recogniziiig this same fact in Ceratia 

 of the Lake of Geneva, and it is one of these specimens that 

 I have drawn in fig. 9. In this individual the skeletogenous 

 membrane covered only a part of the anterior region of the 

 body ; on all the rest of the body it had disappeared, leaving 

 only the vestiges of a right posterior horn and of a small 



* A. Gruber, " Ueber Kerntlieilungsvorgange bei einigen Protozoen," 

 in Zeitschr. fiir wiss. Zool. Bd. xxxviii. 

 t Bergli, loc. cit. p. 214. 



