No. 2.] COMPARATIVE CYTOLOGICAL STUDIES. 



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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 Lepiis and Ovis. A 

 single nucleolus is the rule for Amphioxus, Petromyzon, the 

 birds, and most of the mammals ; the Reptilia, Amphibia, 

 Teleostii, and the SelacJiii have numerous nucleoli. In the 

 Tiinicata there is either a single nucleolus or a nucleolus and 

 paranucleoli ; this is also the rule for the Echinodemiata, 

 Mollusca, and Ajinelida. 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. 



