BIGELOW: EARLY DEVELOPMENT OF LEPAS. 89 
4, Srconp CLEAVAGE. Four CELLS. 
The first cleavage results in the division of the ovum into two cells of 
unequal size; the smaller cell (first micromere ab”), which is anterior in 
position, is largely protoplasmic, whereas the larger, posterior cell (ed?) 
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 daugbter-cell, which 
in each stage will be designated as “ yolk cell.” The addition of the ex- 
ponent indicating the cell generation will prevent 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, ab? 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. 
The second cleavage may take place in the cells ab? 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 (ab*) 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 (ab?) the 
spindle is centrally situated ; the cleavage plane is formed at right angles 
