236 LILLIE. [Vol. XVII. 



they reach the poles, each has reassumed the 6-partite form, 

 i.e., a double rod with two transverse constrictions (PI. XXIV, 

 Figs. II, 12). The second division of the chromosomes for 

 the formation of the second polar globule is longitudinal, as 

 shown in PI. XXV, Figs. 19-21. 



The forms of the chromosomes in the maturation spindles 

 of the Q%g of Unio resemble closely those in corresponding 

 stages of the eggs of Prostkeceraeus (Klinckowstrom, '97), of 

 Thalasse?na (Griffin, '99), and of ZirpJiaea, another lamelli- 

 branch (Griffin, '99). The first division is certainly at right 

 angles to the long axis of the chromosomes, as these lie in the 

 equatorial plate, and the second division is with equal certainty 

 longitudinal. But as the history of the tetrads in Unio is 

 unknown, it is quite possible that the first division is mor- 

 phologically a longitudinal division and the second transverse, 

 that is, a reducing division in the usual sense. Klinckowstrom 

 comes to the conclusion that this is probably the case in the 

 Qgg of Prostheceraeiis, though the evidence he offers is not 

 conclusive. Griffin ('99) has made a very detailed study of this 

 form of chromosome, and has shown in an ingenious manner 

 that the first division may be interpreted as following the 

 longitudinal division of the original spireme, and the second 

 as transverse to this, that is, a reducing division in the ordi- 

 nary sense. If his interpretation should turn out to be just, 

 it would have a wide application, seeing that this form of 

 chromosome is found in flatworms and Gephyrea as well as 

 in mollusks. 



2. Achromatic Structures. 



As already stated, the earliest cytoplasmic phases of matura- 

 tion have not been observed ; thus the question of the origin 

 of the egg-centrosomes will not be considered. We may 

 begin, therefore, with a description of the aster of the first 

 maturation spindle at the time of the metaphase. At this 

 time the aster at either end of the spindle has the following 

 structure (PI. XXIV, Figs. 9 and 10) ; in the exact center is a 

 minute centrosome, which has begun to elongate, subsequent 

 to division. This is imbedded in a substance, homogeneous 



