MUSEUM OF COMPARATIVE ZOOLOGY. 309 



segmentation sphere immediately upon its formation is surrounded by 

 a homogeneous " Plasmazone," and by rays traversing the contiguous 

 granular pi'otoplasm. The rays increase in length as the nucleus, with 

 gradually retarded velocity, moves from the periphery to, or near to, the 

 centre of the egg. The nucleus, at first homogeneous, increases in size 

 after its migration, and becomes less refractive ; the rays and " Plasma- 

 zone " disappear. The nucleus becomes indistinct, and it can be shown, 

 by the use of reagents (p. 213), that it becomes spindle-shaped, streaked, 

 and acquires an equatorial "Zellplatte " [Kern^lsittel]. However, in liviiig 

 eggs (p. 217) one can see that the arrangement of the plasma becomes 

 radial to the two poles of the nucleus about to divide, and more distinct 

 as soon as the segments of the nuclear plate separate, — Auerbach's kary- 

 olitic figure. If, at the beginning of the division, the nucleus remains 

 eccentric, the whole figure moves toward the centre of the egg. The 

 " Kernf aden " [interzonal filaments] are not numerous, and become in- 

 distinguishable before completed division. 



It may be observed in Limax that the filaments remain visible longer. 

 I think their early disappearance in Phallusia may have been due to the 

 nature of the reagent (osmic acid) used by Strasburger, rather than to 

 any difference in the eggs. 



The rays increase in length as the formation of the new nuclei advances, 

 till they reach the periphery and equatorial plane ; the egg lengthens in 

 the direction of the line uniting the new nuclei ; the equatorial plane be- 

 comes clearer. An annular constriction, beginning on all sides at the 

 same time, appears, and the division is accomplished so quickly as to 

 seem simultaneous throughout its whole extent. The rounded segmen- 

 tation products become mutually flattened. 



The nuclear metamorphosis, as demonstrated by reagents, is the same 

 as in Unio (see below). The author believes he has now and then seen 

 slight thickenings (Zellplatte) in the equator of the interzonal filaments 

 similar to those observed in plant cells, and thinks they may arise from 

 the " Hautschichtmasse " collected in the plane of division; he also sug- 

 gests a similar interpretation for Fol's figure of the Pteropod egg, but, as 

 has already been indicated (p. 292), it is probable that the figure in 

 question represents a stage antecedent to the formation of the nuclear 

 plate, and hence can hardly bear the interpretation proposed by Stras- 

 burger. As the second division is being initiated, it sometimes happens, 

 under abnormal circumstances, that the two segmentation cells formed 

 by the first division again fuse. In eggs thus artificially made uni- 

 cellular, there may be distinguished four suns united in pairs by con- 

 necting bands. 



