BIGELOW: EARLY DEVELOPMENT OF LEPAS. 89 



4. Second Cleavage. Four Cells. 



The first cleavage results in the division of the ovum into two cells of 

 unequal size ; the smaller cell (first micromere ah^), which is anterior in 

 position, is largely protoplasmic, whereas the larger, posterior cell {c(F) 

 contains the yolk, and will be designated as " yolk-cell." For conve- 

 nience in description this cell is regarded in the following account of 

 cleavage as a macromere ; it retains its individuality during three suc- 

 cessive unequal cleavages, giving rise to three "protoplasmic " micromeres, 

 the yolk after each cleavage remaining in the larger daughter-cell, which 

 in each stage will be designated as " yolkcell." The addition of the ex- 

 ponent indicating the cell generation will pi-event the confusion which 

 would arise from the use of the term *' yolk-cell " alone, when applied to 

 the cell d^, d^-^ or d^-^, which are the yolk-bearing derivatives of the cell 

 cd^ of the two-cell stage. The micromeres are numbered in the order of 

 their separation from the yolk-cell, aV^ being the first and c^ the second. 



The nearly synchronous successive divisions of the first two cells {ab\ 

 cd^), and afterwards of their derivatives, result in " resting " stages of 

 the egg, which normally consist of 2, 4, 8, 16 and 32 cells, and it be- 

 comes easy to classify the successive cleavages of the egg as second, third, 

 fourth and fifth. It will be noticed, however, that in the second and 

 following cleavages the yolk-bearing cell tends to divide after the other 

 cells, and that its division becomes more retarded at each successive 

 generation. This seems to be correlated with the fact that at each divi- 

 sion the protoplasm in the yolk-cell is diminished in proportion to the 

 amount of yolk. In the fourth and fifth cleavages the yolk-cell usually 

 completes its division just as the other cells prepare for the next cleav- 

 age. However, it is not until after the fifth cleavage (thirty-two cells) 

 that it lags a full generation behind the other cells. The cleavages can, 

 therefore, be classified naturally according to the resting stages, each 

 stage containing twice as many cells as the preceding. 



Tlie second cleavage may take place in the cells aW- and cd'^ simulta- 

 neously (Fig. 28), but either cell may complete the cleavage slightly in 

 advance of the other. In the majority of cases division of the anterior 

 cell iaW) precedes (Fig. 99), but usually the differences in the phases of 

 mitosis in the two cells are very slight. 



In both cells the mitotic spindles for the second cleavage are formed 

 perpendicularly both to the first cleavage spindle (compare Figs. 26 and 

 28) and to the chief axis of the egg. In the first micromere {ah^) the 

 spindle is centrally situated ; the cleavage plane is formed at right angles 



