FERTILIZATION AND CLEAVAGE IO5 



tally into 4 large cells, macromeres, and 4 small cells, micromeres, 

 some 5 minutes before the other quartet (animal) divides meridionally 

 into 8 equal cells, mesomeres (Plate III, Photographs 14, 15). There is 

 thus a definite 12-cell stage preceding the i6-cell stage by about 5 

 minutes at 23 °C. The interval is somewhat variable in different bat- 

 ches of eggs and very rarely (in one batch only out of many hundreds) 

 does not occur at all, the i6-cell stage following directly on the 8-cell. 

 In Lytechinus ( Toxopneustes) variegatus also this fourth cleavage is asyn- 

 chronous according to Tennent (191 1 a) and Tennent, Taylor, and 

 Whitaker (1929), so that there is a definite 12-cell stage. 



In the next cleavage also, the three types of cells, mesomeres, macro- 

 meres and micromeres divide asynchronously, so that there is a 20- and 

 a 28-cell stage preceding the 32-cell stage by 10 and 2 minutes respec- 

 tively (23 °C.). The 4 macromeres divide first, then the 8 mesomeres 

 and then the 4 micromeres. 



It is difficult to follow the division of the different types of cells 

 further, into the 64-cell stage, but sections of later cleavages definitely 

 show that at any particular time the cells are in different stages of 

 mitosis; that asynchrony continues in the later cleavages. There is, 

 however, at least at first, a greater interval between the division of 

 certain sets of cells than between others. There seems to be a major 

 rhythm with considerable spread. 



Recently an abstract on this subject, also for Arbacia, has been pub- 

 lished by Scott and Fox (1952) Their results are similar to mine, and 

 they have carried the asynchrony through the sixth and seventh 

 cleavages. 



It is of interest that many years ago Tennent (1911a) published a 

 detailed table of the stages and times in the development of the Lytechinus 

 egg, similar to the one for Arbacia presented here. He has given the 

 times for asynchronous divisions up to 124 cells. The earlier asyn- 

 chronous cleavages correspond exactly with those of Arbacia. 



The asynchrony in cleavage becomes important in studies of rhythms 

 of oxygen consumption in relation to mitoses, such as those of Zeuthen 

 (195 1, etc.). It may be that in some species the rhythms are more 

 marked than in Arbacia and Lytechinus, and Zeuthen (i 951, p. 52) found 

 that in Psammechinus microtuberculatus "the micromeres have multiplied 

 slower than the rest of the cells." 



m. Micromeres 

 The micromeres of Arbacia are colorless in contrast to the rest of the 



