The Development of Phascolosoma. 141 



fibres are attaclied. This is the completion of the process of longi- 

 tudinal splitting, and is therefore an equation division. Thus, in 

 Phascolosoma, the usual order of maturation divisions is reversed. 

 It is an example of what Kokschelt u. Heider, 1890, have called 

 a "Praereductionstheilung". 



The sperm nucleus with its large astrosphere awaits in the 

 middle of the e.^g the female pronucleus. The aster of the latter, 

 which has been derived from the second maturation spindle, be- 

 comes smaller, but does not completely disappear. In the middle of 

 the enlarged astrosphere of the sperm nucleus, a minute astrosphere 

 appears, which contains the centrosome, and is immediately sur- 

 rounded by prominent curved astral fibres. When the two pro- 

 nuclei meet between the animal pole and the centre of the ^g^, each 

 is accompanied by its own aster. The position of the respective 

 astrospheres, and the difterence in size between the asters which 

 surround them make their identification easy, until equality in size 

 is established between the two. When this occurs, they have already 

 moved to the opposite poles of the plane of contact between the 

 two pronuclei. 



Segmentation of the Egg. The most striking features of 

 the cleavage are: 



I. The large size of the first quartet of „micromeres", which 

 in the three quadrants A, B and C (ventral in the embryo) some- 

 what exceed in size the „macromeres", or blastomeres at the vege- 

 tative pole. 



II. The alternating directions of the spindles of segmentation in 

 successive stages up to 48 cells, and in certain regions of the %gg 

 still further, which accord completely with the usual type of spiral 

 cleavage. 



III. The presence at the active pole in the 48-cell stage of 

 the rosette, cross and intermediate cells, which are characteristic of 

 Annelids, and are represented also in Molluscs and other groups. The 

 rosette and intermediate cells, which are all of nearly equal size, 

 form a Greek cross, which extends across the active (anterior) pole 

 of the ^gg. The „cross cells" are larger than the others just men- 

 tioned, and lie in the angles of this figure. The intermediate cells 

 are formed in the 36-48-cell stage by radial cleavage, which, 

 however, shows a trace of the laeotropic spiral. 



Immediately behind these cells, that are destined to form the 

 apical plate of the trochophore, are the sixteen large „primary" cells 



