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KOFOII): DEVELOPMENT OF LIMAX. 63 



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 tlie 

 division of this quartet connnences with the formation of (f-^ at the 

 twenty-tive-ct^ll stage, b>it 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 Qi. '2, forming 7.3 and 7.4. 



Plate IV. Figs. 31, 33 ; Plate V. Fig. 35 ; 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 

 spindles in all of the cells of this quartet except a^-^, and in tliis the 

 stages preparatory to the formation of the spindle are seen (Fig. 31). 

 The uucleus 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 cell, and closely 

 applied to the nuclear membrane, there are two large, clear spherical 

 spaces in the cytoplasm, bounded by a granular zone. These structures 

 are probably the astrocoels of Fol ('91). The surface of the nucleus pre- 

 sents on one side a peculiar constriction, or crease, running between the 

 two astrocwls, as tliough 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 (Plato 

 IV.) are almost free from any inclination indicative of a spiral arrange- 

 ment. The one in 6^-^ 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^-^, c''^ Figure 35 (Plate V.). The order of nuclear advance- 

 ment in this quartet as shown in Figure 32 (Plate IV.), is h, d, c, a, but 

 in Figure 35 (another eixg) the cell c has been 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 the 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 left spiral ; so also a slight indication of 



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