190 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Of the generations discussed, the fourth and sixth have right, 

 the third and fifth have left spirals. This alternation of spirals 

 apparently rests upon the more fundamental and wide-spread ten- 

 dency of the spindle to take a position at right angles to that 

 of the spindle of the previous division. This tendency can be 

 traced from the last maturation spindle of the ovum through- 

 out the spiral period and in some cases far into the bilateral 

 period of cleavage, as in the development of the mesoderm in 

 Umbrella (Heymons '93). The spiral period of cleavage pre- 

 sents only slight mechanical impediment to the realization of 

 this tendency, which therefore expresses itself here in the alter- 

 nation of spirals. 



The system of nomenclature here employed for Lirnax makes 

 it possible to correlate the two hitherto independent systems of 

 alternation, that of the spirals of the macromeres and that of the 

 micromeres, so often noted by writers on cell lineage (Wilson 

 '92, pp. 378, 391, 439, and '93, p. 600). The alternation is now 

 reduced to a single system, based on generations, which harmo- 

 nizes all of the cleavages of spiral character. 



The law of alternation of spirals in successive generations 

 is, I believe, applicable to all forms of spiral cleavage. At 

 the present time we have two extensive papers of a recent 

 date discussing cell lineage. That of Wilson ('92) carries the 

 discussion through the fifty-eight-cell stage of Nereis, and that 

 of Heymons ('93) through the ninety-one-cell stage of Umbrella. 

 In both of these cases the law of alternation of spirals applies 

 without a single exception, if the spirals are named on the basis 

 proposed in this paper. According to Wilson ('92, p. 439), Conk- 

 lin ('91, '92) has found in the cleavage of Crepidula a close 

 resemblance to that of Nereis. Lillie ('93) has found a corre- 

 sponding agreement in the cleavage of Unio. The law of alter- 

 nation of spirals will then apply to Crepidula and Unio in so far 

 as they conform to the cleavage of Nereis. In Nereis, Umbrella, 

 and Limax we have forms presenting diverse conditions of devel- 

 opment. We have one annelid, and two mollusks of widely sepa- 

 rated genera, one land and two marine forms. In Nereis and 

 Umbrella there is a large amount of yolk of a different nature 

 and distribution in the two forms. They also present free larval 

 stages, with different degrees of precocious development. In 

 Limax there is very little food-yolk, and cleavage is almost equal. 



