168 EDOUARU VAN BENEDEN. 



the disappearance of the vacuole which is seen in these at 

 certain moments. I observed, also, that the very numerous 

 nucleoli in the young ova of the frog likewise execute move- 

 ments which consist in changes of form. I made these 

 observations a little while after having established the 

 alternate disappearance and reappearance of the nucleoli in 

 the nucleus of the Gregarina gigantea} Since then I have 

 several times observed the same fact in the Monocystis 

 lumbricorum. In the latter species, however, there exists 

 one nucleolus of larger size, which is more voluminous than 

 the rest, and which never disappears, but which changes 

 its form, and in which one sees vacuoles appear and dis- 

 appear : sometimes the nucleolus encloses only a single very 

 extensive vacuole ; a few seconds later it may show a crowd of 

 little ones of all dimensions ; at other times there are no 

 longer so many. Other researches and numerous occupa- 

 tions have, however, hindered me from publishing these 

 observations before now. I have not seen changes of form 

 take place under my eyes in the ova of the Starfish ; but I 

 have no doubt whatever that the differences shown to exist 

 in the form of the germinal spot must be attributed to the 

 contractility of the substance of the nucleoli. This con- 

 clusion results from observations made on mature ova of 

 changes which essentially consist in the reduction of the 

 nucleolus into fragments, a reduction which immediately 

 precedes the disappearance of Wagner's spot. 



The pseudo-nucleoli, eight to fifteen in number, are cor- 

 puscles of very variable size, composed of a substance which 

 is much less refractive than the nucleolar matter. Some- 

 times they are disseminated throughout the whole extent of 

 the germinal vesicle ; more frequently they are situated in the 

 neighbourhood of the true nucleolus. They have an entirely 

 different composition, and different properties from the latter. 

 It is incorrect, therefore, to say with Hoffmann that there 

 are from one to ten nucleoli in the ovum of the Asteracanthion 

 rubens. 



Hertwig, in his work, declares himself a partisan of the 

 opinion of Auerbach, who holds that the membrane of the 

 germinal vesicle, and of nuclei in general is produced by the 

 differentiation of a thin layer of protoplasm around a vacuole 

 which is filled with the nuclear substance. Although I do 

 not wish here to go fully into my views on the subject of the 



^ ^^douard van Beneden, " Sur une nouvelle espece de Gregarine, 

 Designee sous le nom de Gregarina gigant<;a " (On a New Species of Gre- 

 garina, named the Gregarina gigantea), ' Bull. Acad. Eoy. de Belg.,' 2nd 

 series, vol. xxviii. See also translation of same in this Journal. 



