KOFOID: DEVELOPMENT OF LI.MAX. 



67 



As stated in my earlier paper ('94, p. 191), there are some indica- 

 tions in Rabl's work that Planorbis presents a case of reversed cleavage, 

 i. e. the cleavage is such that the spirals of the even generations are left, 

 and those of the odd right. Fol's ('80) figures of the eight- and six- 

 teen-cell stages of this Pulmonate (Plate IX. -X. Figs. 1, 2) indicate 

 that the spiral of the fourth generation is a left one, and that of the 

 fifth a right one. My own sketches of Planorbis, made from living 



CLEAVAGE OF CAVOLINA. 



Fol (75). 



eggs in the spring of 1892, show the same direction in these spirals. 

 So far as can be judged from these facts, the alternation of spirals 

 holds in this case, which is probably one of reversed cleavage. 



If the reversed spirals persist until the formation of the mesoderm, this 

 germ layer would then come from the right posterior macromere, instead 



1 Fol does not name or discuss spirals. The nomenclature in this column is 

 deduced entirely from his lettering of the cells. After tlie third generation the 

 divisions of only one quadrant (I) are followed. In this, as in the following tables, 

 capital letters indicate macromeres, and the lineage of only a single quadrant (<i of 

 the revised, nomenclature) is given. 



