DESCRIPTION OF PLATES. 



All the figures of plates l to 6 represent entire eggs of Linergcs mcrcurius, either drawn in the 

 living condition or stained and mounted entire. They were drawn with the aid of the camera 

 lucida under Zeiss apochromatic obj. 8 mm., oc. 6, at table level; in the process of reproduction they 

 were reduced about one-third, so that as they now appear they represent a magnification of about 

 230 diameters. Plates 7 and 8 are photomicrographs of sections, magnified about 175 diameters. 



Plate i. 



Figs, i, 2. Eggs before maturation, showing the peripheral position of the large 

 germinal vesicle. 



Fig. 3. First maturation division ; the maturation spindle is extremely small and 

 apparently without centrosomes or astral rays : at the vegetal pole is 

 a lobe of clear protoplasm which may represent the " yolk lobe " of other 

 animals. The peripheral layer of clear protoplasm is marked off from 

 the deeper-lying substances of the egg by the crenated line and the 

 spherules, which represent yolk. 



Fig. 4. Second maturation division ; the spindle is here smaller than in the first divi- 

 sion ; within a clear area at the vegetal pole a chromatic body is found 

 which probably represents the sperm-head. 



Fig. 5. Egg showing the two germ nuclei side by side. 



Fig. 6. Anaphase of the first cleavage-spindle, showing the asters, chromosomal 

 vesicles, and connecting fibers. 



Pl.\te 2. 



Fig. 7. Appearance of cleavage furrow and "cleavage-head" at the animal pole; 



each of the daughter nuclei consists of two parts, the gonomeres. 

 Figs. 8, 9. Further stages in the formation of the first cleavage furrow by unilateral 



constriction. 

 Figs. 10, 11. Eggs in which the cleavage head turns to one side after reaching the 



vegetal pole. 

 Fig. 12. Egg viewed from the vegetal pole, showing the connecting strand between 



the two blastomeres. 



Plate 3. 



I'iG. 13. Egg showing the cleavage head turned still farther to one side. 



Fig. 14. 2-cell stage from the animal pole, showing the blastomeres flattened against 

 each other. 



Figs. 15-18. Successive stages in the formation of the second cleavage-furrow, show- 

 ing the cleavage-head advancing from the center of the egg toward 

 the periphery. 



Plate 4. 



Fig. 19. Side view of fig. 18, in the line of the first cleavage, showing the arched 

 condition of the egg at the animal pole and its flat appearance at the 

 vegetal pole ; the outlines of the four blastomeres are shown at the 

 animal pole, but the cleavage-furrow has not yet reached the periphery, 

 and hence the daughter cells are still held together here by the con- 

 necting strand. 



Fig. 20. Side view of an egg at the close of the second cleavage, seen in the line 

 of that cleavage; the connecting strands between the daughter cells are 

 shown near the vegetal pole. 



Fig. 21. 8-ccll stage; side view of an egg at the close of the third cleavage, showing 

 at the periphery the connecting strands between daughter cells. 



Figs. 22-24. Eggs showing irregular cleavage, due chiefly to the suppression of the 

 division of the cell bodies ; fig. 24 shows the nuclei and the appear- 

 ance of the delayed cleavages in some of the cells. 



169 



