CELL DIVISION IN EGGS OF CREPIDULA. 589 



PLATE LV. 



Effects of Hypertonic Sea Water. 



All eggs shown on this plate are from Exp. 804, and were subjected to 1 per cent. NaCl in sea water 

 for 4 hrs. 



Figs. 159-163. The sperm nucleus lies in a small area of cytoplasm near the lower pole; the egg 

 nucleus lies in a larger area of cytoplasm at the animal pole; various stages in the formation of the egg spindle 

 are shown. 



Fig. 164. Two spindles, probably those of egg and sperm, are joined at one pole. 



Figs. 165, 166. Tetrasters, probably formed from the egg and the sperm spindles. 



Fig. 167. Two spindles, probably those of the egg and the sperm, joined at one pole. 



Fig. 168. Two spindles, probably those of the egg and sperm, quite separate. 



Figs. 169, 170. Tetrasters in different phases of the separation of the chromosomes. 



PLATE LVI. 

 Effects of Hypertonic Sea Water. 



Fig. 171. Exp. 823: 3 per cent. NaCl, 16 hrs., normal 8 hrs. Egg nucleus very small and clear with 

 chromosomes persistent within vesicle and egg centrosome outside vesicle; the sperm nucleus is enormous 

 and contains much chromatic sap. 



Fig. 172. Exp. 823: Similar to the preceding. 



Fig. 173. Exp. 837: 1 per cent. KC1, 9 hrs., normal 35 hrs. Development has been stopped but the 

 egg is not dead; the germ nuclei are very large and achromatic; small achromatic spherules he in the cyto- 



Fig. 174. Exp. 822: 2 per cent. NaCl, 16 hrs., normal 8 hrs. Achromatin in large and small vesicles 

 in the cytoplasm; germ nuclei normal. 



Fig. 175. Exp. 822: The germ nuclei are broken up into many separate vesicles, each with a chromatic 

 nucleolus. 



Fig. 176. Exp. 823: Two-cell stage from same experiment as figs. 171, 172; nuclei very large and 

 achromatic. 



Fig. 177. Exp. 822: Side view of egg in 2-cell stage, each cell containing a large sphere (S) and one 

 or more chromatic and two or three achromatic nuclear vesicles. 



FiG ; 178, 179. Exp. 810: 3 per cent. NaCl, 15 hrs.: Side views of egg in 2-cell stage showing the 

 chromatin massed within the nuclear vesicle; the latter are elongated along the line of the former spindle 

 axis. 



Figs. 180-182. Same experiment as preceding; 4-cell stages from animal pole. 



Fig. 180. In one half of egg the remains of the 2d cleavage spindle are still visible and the chromatin 

 has formed no nuclear vesicle; in the other half the nuclear vesicles are elongated along the line of the spindle 

 axis, the chromatin being massed at the ends of the vesicle nearest the spheres. 



Fig. 181. Nuclear vesicles elongated along the spindle axis are present in three of the cells, and in 

 each the chromatin is massed at the end of the vesicle nearest the sphere; in the fourth cell no nuclear vesicle 

 is present, but traces of spindle fibres may be seen. 



Fig. 182. In all four cells the nuclear vesicles are rounded and the chromatin is massed near the center 

 of each vesicle. 



PLATE LVII. 



Effects of Hypertonic Sea Water. 



Figs. 183-185 were in 1-cell stage at beginning of experiment; Figs. 186-196 were in 2-cell stage. 



Fig. 183. Exp. 805: 2 per cent. NaCl, 4 hrs. ; in both egg and sperm nuclei the chromatin is aggregated 

 into a dense mass in the center of the nuclear vesicle; there are many cy tasters near the sperm nucleus. 



Fig. 184. Exp. 809: 2 per cent. NaCl, 15 hrs.; the chromatic and achromatic parts of the germ 

 nuclei are in separate vesicles. 



Fig. 185. Exp. 809: Similar to the preceding; a double cytaster is present near the germ nucleus. 



Fig. 186. Exp. 809: 2 per cent. NaCl, 15 hrs.; the chromatic and achromatic parts of the nucleus are 

 in separate vesicles; the achromatic vesicles are numerous and scattered. 



Fig. 187. Exp. 809: Side view of egg similar to preceding. 



Fig. 188. Exp. 809: Egg similar to the preceding showing two principal achromatic vesicles (Ach) 

 and many smaller vesicles or sphere granules. 



Figs. 189,190. Exp. 805: 2 per cent. NaCl, 4 hrs.; the 2d cleavage spindles are much shrunken in 

 size; the archi plasm around them is much condensed while many small radiating masses of archiplasm 

 are left along the astral radiations as cytasters. 



Figs. 191, 192. Exp. 823: 3 per cent. NaCl, 16 hrs., normal 8 hrs.; top and side views of eggs showing 

 in addition to the principal masses of chromatin (Ch) very numerous granules or spherules which probably 

 represent scattered achromatic material (Ach). <; 



Figs. 193, 194. Exp. 837: 1 percent. KC1, 9 hrs., normal 36 hrs.; the numerous karyomeres have 

 begun to absorb achromatin and to fuse together. 



