KOFOID: DEVELOPMENT OF LIMAX. 49 
it is preceded by an elongation of both cells in a direction at right angles 
to the plane of the division. Figure 15 (Plate IL) presents a view 
from the animal pole of a stage preparatory to this cleavage. The egg 
here represented is an exceptionally large one, being about 160 q in its 
longest diameter. Each cell contains a spindle lying in its long diameter 
and nearer the animal pole. If the egg be viewed exactly from the 
animal pole, it is found that two of the asters — the rays of which are 
made more prominent in the figure than those of the remaining two — 
lie at a higher level than their mates. The same fact is brought out in 
a lateral view of this stage (Plate I. Fig. 7). Of the four asters the two 
having the same level lie in diagonally opposite quadrants of the egg. 
If we orient the egg so that the first plane of cleavage is transverse, no 
matter which pair of cells is placed anteriorly, and name the four asters 
A, B, C, D, in the accepted order, beginning at the left anterior quad- 
rant, we shall have the asters A and C at the higher level, B and D at 
the lower. The slight difference in size between the two cells of this 
egg (Plate II. Fig. 15) has been previously noted. "There is also a very 
slight difference in the stage of mitosis exhibited by the two cells, the 
larger being slightly more advanced than the smaller. A difference in 
the time of cleavage of the two cells of this stage has come under my 
observation in Limax a number of times. It is, however, not prevalent, 
and it is impossible to correlate it with any difference in the size of the 
two blastomeres. In Nereis, Umbrella, Cyolas, Unio, and many other 
forms, there is a well marked difference in size and a correlated difference 
in the time of division, the smaller cell being generally the first to divide. 
Figure 8 (Plate L.) represents the second furrow just before its com- 
pletion. The difference in level noted in the asters here finds its counter- 
part in the position of the partially formed blastomeres, the order of 
arrangement being the same as in Figure 15 (Plate II). "The planes of 
division'are perpendicular to the axes of the spindle. They are there- 
fore not continuous, but both are oblique to the vertical axis and in 
opposite directions. The posterior plane (separating C and D) passes 
from above toward the vegetative pole and the right, the anterior (sepa- 
rating A and B) from above toward the vegetative pole and the left. 
Inasmuch as the two derivatives do not lie at the sume level, we may 
test the existence of the spiral; viewing the egg from the animal pole, 
and going from tho lower derivative to the upper, we pass in a direction 
opposite to that in which the hands of a clock move; this oblique posi- 
tion of cognate cells may be referred to as a left spiral. It should be 
noted that this position of the cells is predetermined by the inclination 
VOI , XXVII. — NO. 2, 4 
