The Development of Phascolosoraa. g9 



of the astrosphere, thus connecting the two vesicles. Very prominent 

 astral fibres appear at this stage, radiating outward through the 

 cytoplasm on all sides, except toward the animal pole. Some of 

 these astral rays extend to the periphery of the egg. 



The second polar spindle, in early prophase, lies parallel to the 

 surface of the egg, beneath the ten vesicular chromosomes (Fig. 9). 

 The latter, which have remained in the form of short bilobed masses 

 since the anaphase of the first polar spindle, now become arranged 

 around the middle of the newly formed spindle in the plane of its 

 equator. By the time that the rotation of the spindle into a radial 

 position has been accomplished (Fig. 10), the ten chromosomes have 

 assumed the form of slender U-shaped rods, which lie about the 

 equator of the spindle, their points extending away from it. Near 

 the base of each limb a fibre of the spindle is attached. The bent 

 rods now break apart in the middle, and the two halves of 

 «ach chromosome are drawn toward the respective astrospheres 

 (Fig. 11, 12). The division of the chromosomes in the second polar 

 spindle is to be regarded as the termination of the process of their 

 partial longitudinal splitting in the immature oocyte, by which the 

 chromosomes were transformed from rods into elliptical rings. Since 

 it is longitudinal, the process is an equation division. Thus, as 

 -already stated, the more usual condition of a longitudinal splitting 

 followed by a transverse is exactly reversed in Fhascolosoma, for in 

 this form the transverse or "reducing" division is accomplished before 

 the longitudinal or "equation" division is completed. 



The objection to this conclusion may be raised that nothing is 

 known of the arrangement of the chromatin in the oocyte before 

 the first maturation spindle is formed and the splitting of the 

 chromosomes has begun. The history of the changes in the chromatin 

 previous to this time should indeed be known in order to exclude 

 all possibility of error, but I consider it probable that further in- 

 formation in regard to the changes in the oocyte will not alter 

 essentially this interpretation. 



It is hardly possible, for example, that the chromosomes of the 

 first maturation spindle in Fhascolosoma can have undergone such a 

 series of changes as those described by Geiffin (1899) in Thalassenia, 

 in which the rings or split rods lie at first transverse to the spindle, 

 and become drawn out in the metaphase each into the form of a cross. 

 On the contrary, the chromosomes of the first polar spindle in 

 Fhascolosoma are always rod-shaped or elliptical. They resemble 



