282 



of which they are composed are not separate as in Unio, but are 

 connected together by bridges of a deep staining substance into an 

 irregular, hollow sphere. The irregular, granular form of the centro- 

 some soon gives place to a regular spherical form and at the height 

 of the metaphase each centrosome is a sphere about 1 /t in diameter 

 with a densely staining periphery and a clear center (Dia. A, 4). 



During the anaphase this sphere enlarges rapidly and the dense 

 peripheral layer grows thinner and thinner until it becomes a mere 

 shell inclosing a clear central area. When the chromosomes have 

 reached the spheres at the ends of the spindle the centrosomes are 

 about 4 [.I in diameter and each incloses in its central clear area a 

 faintly staining central corpuscle, which has the form of a hollow 

 sphere (Dia. A, 5). In still later stages the peripheral layer of the 

 centrosome breaks up into a number of pieces or plates which are 

 finally resolved into a layer of granules (Dia. A, 7, 8); meanwhile 

 the hollow central corpuscle becomes elliptical in shape and a dark 

 granule appears at each pole of the ellipse (Dia. A, 6, 7, 8). These 

 granules are the centrosomes of the second maturation division and 

 the remainder of the ellipse becomes the central spindle. Both cen- 

 trosomes and central spindle of the second maturation arise there- 

 fore, within the centrosome left in the egg at the close of the first 

 maturation; finally the layer of granules which marks the periphery 

 of the later completely disappears leaving the second polar amphiaster 

 free in the cytoplasm. The general resemblance between these cen- 

 trosomes and those described by Mac Farland ^), Lillie '), Vejdows- 

 KY^) and Mrazek and Van der Stricht^) is at once apparent, 

 though there are certain important difi'erences in details. 



The history of the centrosomes of the second maturation is wholly 

 similar to that of the first. In the prophase they are minute granu- 

 les which become irregular in shape, then spherical, then hollow 

 spheres. After the second polar body has been cut off the hollow 

 centrosome left in the egg remains visible until after the egg nucleus 

 is fully formed. It is filled with a mass of small granules, none of 

 which look like the central corpuscles of the preceding division. 



1) Mac Farland, Celluläre Studien an Molluskeneiern. Zool. Jahrb., 

 Anat., Bd. 10, 1896. 



2) LiLLiE, loc. cit. 



3) Vejdowsky and Mrazek, Centrosom und Periplast. Sitzungsber. 

 böhm. Ges. Wiss. Prag, 1898. 



4) Van der Stricht, La formation des globules polaires, etc. chez 

 Thysanozoon. Arch. Biol., T. 15. 



