KARYOKINESIS. 17 



fore, the centrosomes, like the chromosomes, undergo an unusual type of division. 

 The centrosome which goes into the second polar body completely disappears. 



The egg centrosome is surrounded by a sphere with grows with the growth of 

 the centrosome until it becomes very large and is filled with faintly staining granules 

 which are held in a delicate reticulum, fig. 36. In some cases the outlines of this 

 sphere are very distinct, in others more faint, but in all cases there are strongly 

 marked polar fibres, which run from the periphery of the sphere for a considerable 

 distance through the egg ; some of these fibres may also be traced into the spheres 

 where they appear to break up into rows of granules. In stages later than fig. 36 

 I am unable to recognize the egg centrosome ; its granules merge with those of the 

 surrounding sphere and its outlines are no longer visible. 



In structure and metamorphosis the centrosomes of the maturation divisions of 

 Crepidula are very similar to those described by MacFarland ('97) in Diaulula, 

 Lillie ('98) in Unto, Vejdovsky and Mrazek ('98) in Rhynchelmis and Van der 

 Stricht ('98) in Thysanozoon. The further consideration of these centrosomes will 

 be deferred until after the description of the cleavage. 



3. Polar Rats, Spindle Fibres and Spheres. — At the time of the entrance 

 of the spermatozoon into the egg, figs. 3 and 4, the centrosomes are surrounded by 

 polar fibres and the nuclear membrane is indented in the region of each centrosome. 

 A large number of fibres, forming a cone or half spindle, can be traced from each 

 centrosome to the indented portion of the nuclear membrane. Within the nucleus 

 the linin threads, with their attached chromatin granules, are arranged in the same 

 radiating lines as these fibres and form an intranuclear continuation of them, figs. 3 

 and 4. 



In early stages of this division both the extra- and intra-nuclear portions of the 

 spindle consist of branching and anastomosing threads, along which are ranged 0x3-- 

 chromatin granules. These two groups of fibres, i. <?., those inside and those out- 

 side of the nucleus, are so essentially alike that I cannot doubt that both are derived 

 from the same substance, viz., the linin and oxychromatin of the nucleus, in which 

 case the extra-nuclear portion of the spindle must be formed from nuclear constitu- 

 ents which have escaped from the nucleus at the indented areas mentioned above. 



The polar fibres also consist of threads along which granules are attached, figs. 

 4, 5 and 6. In the first maturation they closely resemble the spindle fibres, and, 

 like them, may be derived from the achromatic nuclear substance. These granules 

 are rarely arranged in concentric spheres as Heidenhain, Driiner and Lillie have 

 found them. As mitosis advances, both polar 'and spindle fibres become smooth. 

 There can scarcely be any doubt that this is accomplished by the transformation of 

 these granules into the substance of the fibre (cf. Boveri, '88, p. 80, Wilson '95, 

 Griffin '99). Again, in the dissolution of the spindle one frequently observes that 

 the fibres become varicose, as in the early stages of mitosis, while the portion of a 

 fibre between granules becomes less and less prominent, figs. 16<z and 33. I have 

 been unable to observe these same varicosities on the fibres of the central spindle in 

 the maturation divisions, but they can be seen in the central spindles which are 

 found during the cleavage of the egg (Plate IV, figs, 74, 75 and 76, and text fig. IV). 



3 JOUEN. A. N. S. PHILA., VOL. XII. 



