CELL DIVISION IN EGGS OF CREPIDULA. 587 



Fig. 112. Exp. 1016: Eggs placed for 27 hrs. in sea water which had been boiled and cooled; similar 

 to preceding, development stopped. 



Fig. 113. Exp. 1017: Eggs left for 48 hrs. in stoppered test tube of boiled and cooled sea water; 

 development completely stopped; sphere granules prominent. 



Fig. 114. Exp. 1016: Same as fig. 112; nuclei with little chromatin; sphere granules prominent. 



Fig. 115. Exp. 1023: Eggs subjected to atmosphere of hydrogen for 2 hrs. and then left in open bottle 

 for 2 hrs.; development stopped; eggs similar to all others subjected to decreased oxygen tension. 



Fig. 116. Exp. 1017: Same as in fig. 113: The spindles are small, deep-staining and without astral 

 rays; the chromosomes are arranged in a ring around the spindles. 



Figs. 117-118. Exp. 1025: Eggs left for 18 hrs. in stoppered bottle of sea water through which hydrogen 

 had been run for 1 hr.; eggs similar to fig. 115; in fig. 118 there is an area of yolk (Y) around the upper 

 poles of the spindles. 



PLATE LII. 

 Effects of Carbonic Acid. 



Eggs in 1-8-cell stage left for 28 hrs. in sea water J saturated with C0 2 . 



Fig. 119. Exp. 1163: Egg showing the individually distinct chromosomal vesicles of the maturation 

 divisions; also the cell membrane separated from the subjacent egg substance. 



Fig. 120, Exp. 1163: Eggs showing five protoplasmic cells (one of them with four nuclei) on the 

 unsegmented yolk. 



Fig. 121. Exp. 1164: Side view of an egg similar to the preceding. 



Fig. 122. Exp. 1163: The second polar body is abnormally large; an accessory aster (<S) lies in the yolk; 

 the division of the nucleus in the 1st cleavage has taken place normally, though the spheres are prevented 

 from moving to their normal positions above the nuclei by the presence of the large polar body; the cleavage 

 furrow cuts into the egg from the animal pole side only, and ends in a " cleavage head " as in ccelenterate eggs. 



Fig. 123. Exp. 1163: The C and D quadrants are entirely normal; in the blastomere AB, the nucleus 

 divided but the cell did not; these two nuclei in an undivided cell gave off a single blastomere of the first 

 set (lab) with large lobulated nucleus, the nuclei, spheres and cytoplasmic areas of the cell AB then came 

 to he at opposite sides of the macromere and each divided independently giving rise to a second micromere 

 (2a, 26) which is nearly normal. 



Fig. 124. Exp. 1163: There are three macromeres one of which contains a perfect spindle but no 

 chromatin; another contains a polyaster and has given off a large micromere with three nuclei; the third 

 contains a single nucleus which is smaller than normal and has given off in reversed cleavage a 1st and a 2d 

 micromere, the former of which is dividing. 



Fig. 125. Exp. 1163: The first cleavage furrow failed to appear though the nuclei divided; at the 2d 

 cleavage each nucleus with its adjacent ooplasm gave off a smaller macromere (B and D), leaving macromeres 

 A and C still undivided; each of these four macromeres has given off a micromere of the 1st set. 



Fig. 126. Exp. 1163: Three macromeres one of which contains several centrosomes and spheres 

 (S) but no nucleus; another contains a bifurcated spindle and a normal one and has just given off two micro- 

 meres one on the right and one on the left; the other macromere contains a single nucleus and sphere and has 

 just given off a micromere on the right; two large micromeres with abnormal nuclei occupy the center of 

 the micromere field. There is a general resemblance of this egg to that shown in fig. 124. 



Fig. 127. Exp. 1163: Side view of egg with two macromeres and several micromeres. Strands of 

 protoplasm run from the polar bodies to the micromeres and from one of the latter to a macromere, sug- 

 gesting the "spinning" activities of other eggs; lobes are also found on several cells. 



Fig. 128. Exp. 1163: Two macromeres, each with two nuclei and two or more spheres; in the second 

 cleavage the nuclei divided but the cell-body did not; in the third cleavage there was probably a triaster 

 in each macromere, since only two micromeres of the first set were formed (lab, led) each with multiple 

 nuclei; in the fourth cleavage each macromere contained two separate spindles and gave off two separate 

 micromeres (2a, 2b, 2c, 2d) of the second set; the nuclei in the macromeres are so well separated that it is 

 probable that at the next cleavage two independent spindles would form in each macromere and would 

 lead to the formation of four micromeres of the third set. 



Fig. 129. Exp. 1163: Macromeres A and B did not separate at the 2d cleavage, but each has given 

 rise to three micromeres forming a typical micromere plate, though the direction of division in A and B 

 has sometimes been atypical. 



Fig. 130. Exp. 1164: Irregular cleavage mass in which it is not possible to identify many cells. 

 Several of the cells show loose membranes and lobes. 



Fig. 131. Exp. 1163: One of the macromeres (D) was separated from the other three, but each has 

 given rise to three micromeres which have subdivided in normal manner, the micromeres formed from D 

 lying on the right of the micromere plate formed from the other macromeres. 



PLATE LIII. 



Effects of Diluted Sea Water. 



In figs. 132-135, 137-140, 144 the dilution was one part sea water to two parts fresh water; in all 

 other cases the sea water was diluted with equal parts of fresh water. With higher dilutions the blasto- 

 meres tend to separate but do not swell appreciably. 



