3o8 MEAD. 



EXPLANATION OF PLATE X. 

 Ainphitrite ornata Verrill. 



Fig. I. Unsegmented egg, viewed from animal pole, showing first cleavage 

 spindle inclined to future sagittal plane of embryo (line joining x and y), and 

 nearer one end of the egg ; / and r, left and right ; x, ventral, y, dorsal ; tn, egg 

 membrane. 



Fig. 2. The 2-cell stage, viewed from vegetative pole : AB and CD, first two 

 blastomeres. 



Fig. 3. Viewed from one end : shows the spindle of first two blastomeres 

 inclined to meridian at an early period ; p-g., polar globules ; a«., animal pole ; 

 veg., vegetative pole. 



Fig. 4. Division of first two blastomeres nearly completed, viewed from 

 vegetative pole : A, B, C, and D, first four blastomeres, the anlagen, respectively, 

 of the left, ventral, right, and dorsal quadrants. 



Fig. 5. The 4-cell stage, viewed from animal pole : karyokinetic figures not 

 all of same age ; /-/, first cleavage furrow ; //-//, second cleavage furrow. 



Fig. 6. Side view : division of the four cells into eight. 



Fig. 7. The 8-cell stage from below (vegetative pole) : a, b, c, and d, the four 

 upper cells which constitute the umbrellar or upper hemisphere ; Ai, Bi, Ci, and 

 Di, four lower cells which constitute the subumbrellar or lower hemisphere ; cr./., 

 cross-furrow ; seg.cav., segmentation cavity. 



Fig. 8. From animal pole : transition from 8-cell to i6-cell stage. The 

 lowermost products of the division of the four uppermost cells are the primary 

 trochoblasts a'-', ^'•', <:'•■, and cT-^. 



Fig. 9. From lower pole : transition from 8 to i6-cell stage. 



Fig. id. From left side : transition from 8 to i6-cell stage. 



Fig. ii. i6-cell stage from above: nearly all the cells contain karyokinetic 

 spindles in early stages which indicate the direction of the next cleavage. 



Fig. 12. Same egg as Fig. 11 from right side : seg.cav., segmentation cavity. 



Fig. 13. Same egg as Figs. 11 and 22 from left side: the spindles in ^m, <;'■', 

 d'^, and D^ are seen from the end ; the spindle in a"' is in an abnormal position ; 

 it is in all other eggs I have examined parallel with that in o'l.i. 



Fig. 14. Same egg from vegetative pole. 



Fig. 15. From vegetative pole: later than last figure; transition to 32-cell 

 stage. 



Fig. 16. 32-cell stage from animal pole : the furrows drawn with heavy 

 lines, //-//, indicate the direction of the second cleavage furrow ; those in dotted 

 lines, /-/, that of the first cleavage furrow. 



