588 CELL DIVISION IN EGGS OF CREPIDULA. 



Fig. 132. Exp. 875: Second polar spindle at animal pole; sperm nucleus has formed a spindle (tfSp); 

 the homogeneous chromatic sphere below this may represent an accessory sperm nucleus (<?N). 



Fig. 133. Exp. 875: First cleavage spindle; the seven chromatic spheres may represent accessory 

 sperm nuclei (cf N). 



Fig. 134. Exp. 875: Enormous second polar body containing large nucleus and yolk; two nuclei and 

 accessory sperm nucleus ( <?N) in egg. 



Fig. 135. Exp. 875: Probably i blastomere containing polyaster and with a micromere which has 

 just divided. 



Fig. 136. Exp. 872: Three blastomeres showing reversed polarity, the spheres, nuclei and cytoplasmic 

 areas lying at the pole opposite the polar bodies; one sphere is found in each cell but in the two larger ones 

 the nuclei are multiple. 



Fig. 137. Exp. 875: Two macromeres, one containing a triaster, the other a tetraster; the two micro- 

 meres are normal except for their large size. 



Fig. 138. Exp. 875: Similar to the preceding. 



Fig. 139. Exp. 875: Side view of an egg similar to figs. 137, 138. 



Fig. 140. Exp. 875: Macromeres A and B have not divided and the chromosomes are irregularly 

 scattered in the spindle; macromeres C and D have divided normally giving rise to first and second micro- 

 meres and the first set are subdividing normally. 



Fig. 141. Exp. 859: Chromosomes were scattered along the spindle during the third cleavage and 

 have given rise to chromatic connections between daughter nuclei, which resemble amitoses. 



Fig. 142. Exp. 872: The micromeres are larger than usual (two of them contain yolk) and they have 

 caused a separation of A, B, from C, D. 



Fig. 143. Exp. 872: The micromeres are larger than usual and contain yolk; the macromeres are 

 separated and one which has just divided (Y, Y) contains yolk but no cytoplasmic areas; the chromosomes 

 are here scattered along the spindle axis, thus forming a chromatic connection. 



_ Fig. 144. Exp. 875: f blastomeres, each of which has given rise to one micromere, which has sub- 

 divided; the macromeres contain spindles along which the chromosomes are scattered irregularly. 



PLATE LIV. 



Effects of Diluted Sea Water. 



In all experiments represented on this plate sea water was diluted with equal parts of distilled water. 



Fig. 145. Exp. 858: Several micromeres have been formed but the yolk has not divided; three of the 

 cells contain several nuclei and spheres, the result probably of polyasters, and one contains a tetraster. 



Fig. 146. Exp. 993 (1): Similar to the preceding; the protoplasmic micromeres are partly constricted 

 from the yolk. 



Fig. 147. Exp. 858: Exogastrula; similar to the preceding but of a more advanced stage; the multi- 

 nucleate yolk cell is uncovered by the ectoderm. 



Fig. 148. Exp. 993 : Similar to the preceding. 



Fig. 149. Exp. 993: Side view of an egg placed in diluted sea water in the 2-cell stage; the second 

 cleavage of the yolk was suppressed, but several micromeres have been formed from each macromere. 



Fig. 150. Exp. 956: Egg similar to the preceding, viewed from the animal pole; each macromere 

 contains a large quadripartite nucleus and has given rise to twelve micromeres, which cannot be individu- 

 ally identified. 



Fig. 151. Exp. 993: Isolated i blastomere, the yolk cell has not divided, but contains several nuclei 

 and has given rise to nine micromeres. 



Figs. 152, 153. Exp. 858: Top and side views of eggs which were placed in diluted sea water after 

 formation of the 1st set of micromeres; several dividing cells contain triasters or tetrasters and the chromo- 

 somes are widely scattered; chromatic connections between daughter nuclei are falsely suggestive of 

 amitosis. 



Fig. 154. Exp. 858: Side view of egg placed in diluted sea water after formation of the three sets of 

 micromeres which are approximately normal; scattered chromosomes have given rise to chromatic con- 

 nections between daughter nuclei. 



Fig. 155. Exp. 993: Isolated f blastomeres which have produced a f micromere plate; the macro- 

 mere 4D has given off the mesentoblast id which is now dividing in normal manner. 



Fig. 156. Exp. 871 : The 1st set of micromeres have divided twice in normal directions, as indicated 

 by the arrows, giving rise to twelve micromeres; in the formation of the 2d set of micromeres the division 

 of the macromeres was approximately equal. 



Figs._ 157, 158. Exp. 858: Eggs in which the nuclear division at the 2d cleavage took place normally, 

 but in which the cell body did not divide; three quartets of micromeres were formed and have subdivided 

 in approximately normal manner, although there are only two separate macromeres. The 4th quartet 

 cells 4d and 4c form simultaneously from the undivided macromere CD, though in normal eggs 4d (the 

 mesentoblast) forms at the 24-cell stage, while 4c (an entoblast) does not form until the 52-cell stage. 



