No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 
Form. 
A Single More than One Nu- Nucleolus and 
Nucleolus. cleolus, All Alike. Paranucleolus. 
{ 
Gallus 
Fringilla 
Columba 
Felis 
Cavia 
Mus 
Vespertilio 
Sus 
Myoxus 
Talpa 
Ovis 
Lepus 
Homo 
* (Holl) 
* (v. Wittich) 
* (mihi) 
* (St. George) 
* (Rein) 
* (Holl) 
* (v. Beneden) 
* V (Leydig) 
* (Nagel) 
* (St. George) 
* (Flemming) 
nor yet upon the mode of deposition of the egg ( i.e ., whether 
it is pelagic, hatched in a cocoon, or nourished in an uterus). 
These facts hardly warrant an attempt to explain the factors 
limiting the number of nucleoli, and perhaps such explanations 
should rather be expected from experimental workers than from 
purely structural observers. On examining the metazoan groups 
in detail we find in certain of them a degree of uniformity in 
regard to the number of nucleoli. Thus the only vertebrate 
ova with two kinds of nucleoli are those of Lepus and Ovis. A 
single nucleolus is the rule for Amphioxus , Petromyzon , the 
birds, and most of the mammals ; the Reptilia , Amphibia , 
Teleostii, and the Selachii have numerous nucleoli. In the 
Tunicata there is either a single nucleolus or a nucleolus and 
paranucleoli; this is also the rule for the Echinodermata , 
Mollusca, and Annelida. In the Arthropoda there is consider¬ 
able diversity in regard to the number and differentiation of the 
nucleoli. In the nemerteans we find most usually either a 
single nucleolus or a large number of small ones. In the 
Plathelminthes one or two is the rule ; this is also most 
frequently the case for the coelenterates, but in some of the 
latter paranucleoli have been described. 
