No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 285 



lus is absent, in others there is a single one, sometimes with 

 a vacuole; in the Choanoflagellata there is always one large, 

 spherical nucleolus, in the Cystojlagcllata several of various 

 sizes ; and in the Dinojiagcllata there may be several small 

 nucleoli, which are sharply localized from the chromatin, but 

 show the fine reticulation of the latter element. In the Ciliata 

 and Suctoria there are nucleoli of varying size and number in 

 the macronucleus, but none in the micronucleus. 



Chun ('80) iinds in the egg of all Ctenopliora a single large 

 nucleolus, very rarely two. 



Engelmann ('80) figures the nucleoli of certain ciliated cells 

 of various invertebrates as each surrounded by a clear space, 

 the outer boundary of which is marked on optical cross-section 

 by a circle of granules. 



Flemming ("So) concludes in regard to the nature of the 

 nucleolus : " Dass die Nucleolen iiberhaupt keinerlei mor- 

 phologischen Antheil an der Kernvermehrung nehmen"; and 

 " Dass die Dinge, die wir Nucleolen nennen, vielleicht gar keine 

 morphologisch wichtige Theile des Kerns sein mogen, sondern 

 nur Ablagerungen von Substanzen, welche fiir den Stoffwechsel 

 im Kern verbraucht und wieder neugebildet werden; sie wiirden 

 damit gewiss physiologisch wichtige Theile des Kerns bleiben, 

 — was ohnehin durch ihr f ast allgemeines Vorkommen bewahrt 

 wird, — aber doch keine eigentlich organischen, d. h. morpholo- 

 gisch-wesentlichen Kernbestandtheile." 



O. Hertwig ('80) found in the eggs of Chaetognatha numerous 

 small nucleoli. 



Shafer ('80), ovum of Callus: there is a single nucleolus, 

 which in young germinal vesicles consists of a homogeneous 

 matrix which stains slightly with haematoxylin, and a number 

 of coarse granules which stain deeply ; in larger ova the 

 nucleolus is homogeneous throughout and stains deeply. The 

 threads radiating from the periphery of the nucleolus may be 

 either artefacts or may be regarded as extrusions of the homo- 

 geneous substance of the nucleolus. Ovum of Lepiis: in 

 younger nuclei the nucleolus has the same general structure 

 as in the fowl, though it is more irregular in form. In some 

 larger ova the nucleolus " is represented by a number (a dozen 



