21-i PROCEEDINGS OF THE AMERICAN ACADKMY. 



EXPLANATION OF PLATES. 



Arrows joining cells indicate divisions tiiat have occurred since the last stage 

 figured or described. 



A = Anterior. P — Posterior. R — Right. L = Left. D = Dorsal. V = 

 Ventral. For the system of lettering employed to mdicate cell lineage, consult 

 the text, pp. 203, 204. 



PLATE 1. 



Ciona intestindlis. 



All the figures of this plate are drawn to the same magnification, 400 

 diameters. 



Figure 1. Egg of Ciona, 24-cell stage, ventral view. The cells of the ventral 

 half of the egg number sixteen, and are all in the sixth generation. 

 The most recent division is seen to have been that resulting in the 

 posterior cells D»-3, D"^-"*, and C«-3, C*>i. 



" 2. The same egg, dorsal view. The cells of the dorsal half of the 

 egg are still in the fifth generation, and consequently number 

 only eight. They are, however, about to divide, and the spindles 

 indicate the directions in which divisions will occur. 



" 3. Egg of Ciona, 32-cell stage, ventral view. (Divisions which were 

 already completed at the 24-cell stage are again indicated by 

 arrows, for convenience in comparison.) 



" 4. The same egg, dorsal view. 



" 5. Ess of Ciona, 46-cell stage, ventral view. All the cells of the 

 ventral half of the egg are seen to have divided more or less 

 completely since the 32-cell stage, except the small posterior cells 

 C6-3 and W'^. 



" 6. The same egg, dorsal view. All the cells of the dorsal half of the 

 egg are still in the sixth generation, as in Figure 4. 



