KOFOID : DEVELOPMENT OF LIMAX. 49 



it is preceded by an elongation of botli cells in a direction at right angles 

 to the plane of the division. Figure 15 (Plate II.) 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 1 GO jit 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 tlie figure than those of the remaining two — 

 lie at a higher level than their mates. Tlie 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. Tlie sliglit difi'erence in size between the two cells of this 

 egg (Plate II. Fig. 15) has been previously noted. There is also a very 

 slight ditference 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 cori'elate it with any difference in the size of the 

 two blastomeres. In Nereis, Umbrella, Cyclas, Unio, and many other 

 forms, there is a well marked difference in size and a correlated difference 

 in the time of division, the smaller ceW being generally the first to divide. 

 Figure 8 (Plate T.) 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 same level, we may 

 test the existence of the spiral ; viewing the eg^^ from the animal pole, 

 and going from the 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 i*eferred to as a left spiral. It should be 

 noted that this position of the cells is predetermined by the inclination 



VOL. XXVII. — NO. 2. 4 



