No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 373 



Nomenclature. 



In the matter of nomenclature I have followed the system 

 used by Conklin in his paper on the "Embryology of Crepidula," 

 as this method enables one readily to follow the type of cleavage 

 found in annelids and mollusks. Besides, the comparisons con- 

 stantly to be made throughout this paper, with the work of 

 Dr. Conklin, render it highly desirable, aside from other con- 

 siderations, that the same system of nomenclature be em- 

 ployed. The word "quartette" is used to designate the products 

 of a generation of cells given off from the four cells at the 

 vegetal pole of the Q.gg. The term "quartette" has been used, 

 however, in a different sense by several writers, who employ 

 it to designate any four cells of radially symmetrical origin. 

 Thus, according to the latter usage, the four outer cells arising 

 from the division of the iirst generation of ectomeres would con- 

 stitute one quartette, and the four apical cells another, while, 

 according to the usage here employed, all of the eight cells 

 resulting from this cleavage would still belong to the same 

 quartette. As a substitute for the word "quartette" in the 

 latter sense, the word "tier" has been employed; thus the 

 products of cleavage of the first generation of ectomeres would 

 be called the upper and lower tiers of the first quartette. 



The different quartettes in Conklin's scheme are designated 

 by coefficients, and the genealogy of the cells of a quartette is 

 indicated by exponents. The upper cell, or the right one when 

 the cleavage is equatorial, is indicated by the smaller exponent; 

 2^', for instance, indicates the upper cell in the a quadrant of 

 the second quartette, 2a' the lower. The upper product of the 

 cleavage of 2«' would be 2^''', while 2a^'^ would represent the 

 lower cell. 



The Eggs and Egg Masses. 



The egg masses of Planorbis are flattened and rather firm, 

 and are usually found adhering to stones or aquatic plants. The 

 eggs proper are found in relatively large capsules, which are 

 imbedded in a jelly-like mass, outside of which is a somewhat 

 tough enclosing membrane. The amount of jelly between the 



