KOFOID: DEVELOPMENT OF LIMAX. 13 
formed synchronously with the other members of the quartet to which 
it belongs. In this respect Limax stands in sharp contrast to Nereis, 
where the primary mesoderm cell originates at the thirty-eight-cell stage, 
but the cleavage of the remaining cells of the quartet is long delayed. 
Also in Umbrella there is a corresponding lack of synchronism, for the 
division of this quartet commences with the formation of d"? at the 
twenty-five-cell stage, but is not completed until the forty-seven-cell 
stage is reached, Likewise in Planorbis the formation of the primary 
mesoblast antedates the cleavage of the other cells of the same quartet. 
Division of Quartet 6.2, forming 7.3 and 7.4. 
Plate IV. Figs. 31, 32; Plate V. Fig. 355 Plate VI. Figs. 38, 40. 
The spindles resulting in this division are among the first to appear 
in the twenty-four-cell stage. Figures 31 and 32 (Plate IV.) show 
92 and in this the 
spindles in all of the cells of this quartet except a 
stages preparatory to the formation of the spindle are seen (Fig. 31). 
The nucleus is very large; the chromatic granules are distributed in a 
network, and the nuclear membrane is still intact. At diametrically 
opposite sides of the nucleus, in the long axis of the coll, and closely 
applied to the nuclear membrane, there are two large, clear spherical 
spaces in the cytoplasm, bounded by a granular zone. These structures 
aro probably the astrocools of Fol (91). The surface of the nucleus pre- 
sents on one side a peculiar constriction, or crease, running between tho 
two astroceels, as though they were connected by a strand of substance 
(central spindle) which was compressing the thin nuclear membrane, 
The spindles in this quartet, as shown in Figures 31 and 32 (Plate 
IV.) are almost free from any inclination indicative of a spiral arrange- 
ment. The one in 0% shows traces of a right spiral, but there is indica- 
tion from the position of the spindles that the division will be equatorial 
rather than oblique ; such indeed is the character of the division, as is 
shown in c%, ec, Figure 35 (Plate V.). The order of nuclear advance- 
ment in this quartet as shown in Figure 32 (Plate IV.), is b, d, c, a, but 
in Figure 35 (another egg) the cell e has boen the first to divide, whereas 
in Figure 38 (Plate VI.), a forty-cell stage, the cell d?? is just dividing, it 
being the last of its quartet to undergo tho process. This seems to show 
either that the cleavage in this quartet progresses very slowly, or, what 
is more probable, that there is considerable variation in the sequence in 
which its components divide. In the case of Figure 38 (Plate VI.) 
there is strong indication of a Zeft spiral; so also a slight indication of 
