192 PROCEEDINGS OP THE AMERICAN ACADEMY. 



ceases to be contradictory when the author's figures are relabelled, 

 and thereby become better reconciled with each other as well as 

 with the principle of the alternation of spirals. One such case is 

 found in Lang's work on Discocoelis ('84, Taf. XXXV. Fig. 6), and 

 another in Blochmann's paper on Neritina ('81). With the lat- 

 ter I shall deal in this paper. It is only fair to note that Bloch- 

 mann's work was that of a pioneer in the field of cell lineage, 

 and it is therefore not strange that later observers, in the light of 

 comparative study, find their results at variance with his ; as Wil- 

 son and Conklin have in reference to the origin of the " cross " 

 (Wilson, '92, p. 441). I shall take great liberties with Bloch- 

 mann's work, and shall endeavor to show that the cleavage of 

 Neritina, as figured by him, conforms to the law of alternation of 

 spirals, and shall give such arguments as I can from internal evi- 

 dence and theoretical considerations to support my interpretation ; 

 but it is to be remembered that my conclusions remain hypotheti- 

 cal until verified or disproved by renewed observation. Figures 

 7-12 (Plate II.) are reproductions in outline respectively of Fig- 

 ures 45-48, 50, and 51 (Taf. VII.) of Blochmann's paper ('81), with 

 his labelling outside the limits of the figures and my own inside 

 the same limits. No exception is taken to his interpretation until 

 the stage of Figure 10 is reached. Here I must differ radically 

 from the author's interpretation of the relations of the two quar- 

 tets a 3 -d 3 and a 2 -d 2 l . The designations of the cells a 3 , d 2 , d 3 , 

 c%, c 3, W, h> «2 T > must all be shifted one place to the right ; 

 i. e. in the direction of the hands of the watch, as indicated by 

 the long arrows outside the limits of the figure in Figure 10. In 

 my interpretation the quartet a 3 -d 3 is the quartet designated as 

 a 2 -d 2 , and vice versa. The change is made upon the following 

 grounds. 



(1.) The spindle in the cell b, for example, Figures 8, 9, indi- 

 cates that the cell b 3 (Fig. 8, == my rf 6 - 2 ) will lie above and to the 

 right of the cell b (my rf 6 - 1 ), and perhaps higher (Figs. 9, 10) than 

 the upper derivative (my f Z 6 - 4 ) of the cell b 2 (my d 5 - 2 ), and that it 

 will lie in contact with that derivative (d eA ) and with the lower 

 derivative (r< 6 - 3 ) of the cell a 2 (my a 5 - 2 ). This is my interpreta- 

 tion of the position of the cell b s , as foreshadowed by the spindles 

 of Figures 8, 9. However, in Figure 10 Blochmann lias placed 

 the cell b 3 on the left side of the cell b, and in contact with 

 a 2 l and b 2 , a position directly contradictory to that indicated 

 by the position of the spindle of the cell b in Figures 8, 9. 



