250 LILLIE. [Vol. XVII. 



formation of the chromosomes of the first cleavage spindle 

 (PI. XXVI, Figs. 41-43). What becomes of the chromatin 

 which does not pass into the chromosomes .-' If the basichro- 

 matin granules, which stain so intensely in haematoxylin, were 

 thrown out of the nucleus as such, it would be an easy matter to 

 follow them, but apparently they are first transformed into oxy- 

 chromatin, which does not stain in haematoxylin, and in this 

 condition they are found on the mantle-fibers of the spindle, 

 which differentiate out of the intranuclear network (PI. XXVI, 

 Fig. 42). However, some of the unaltered basichromatin is 

 found near the spheres (PI. XXVI, Figs. 42 and 43). A great 

 part of the chromatin, then, is probably transformed into 

 spindle fibers, as Wilson has maintained in the case of the 

 sea-urchin ; and a smaller part probably enters into close 

 relations with the sphere substance, which it may influence 

 through transfusion or other mode of transference. 



The germ-nuclei never "fuse" to form a "segmentation 

 nucleus," but from each arises a group of chromosomes. The 

 chromosomes are at first long and narrow, and longitudinally 

 split from the time of their first appearance (PI. XXVI, Figs. 

 42 and 43). Later they shorten and condense, staining more 

 intensely, and become short, bent rods (PI. XXVI, Fig. 44, 

 and PI. XXVII, Fig. 45). 



2. Literature afid Theory of Fertilization. 



It is well known that, in the majority of animals in which 

 the process of fertilization has been studied with sufficient 

 care, the cleavage centers arise from the sperm-amphiaster. 

 (See Boveri, '87, '95; Crampton;^ v. Erlanger, '97, '98; Fick, 

 '93; Griffin, '96, '99; Hill, '95; Korschelt, '96; Kostanecki, '96; 

 Mead, '95, '98; Sobotta, '97; Van Der Stricht, '97; Wilson and 

 Mathews, '95.) When I began the study of the Qgg of Unio I 

 had no reason to anticipate any other result than this, and had 



^ Crampton's work on the fertilization of the egg of Molgula (an Ascidian) is 

 not yet published, but, as it has been ready for some time, the author kindly per- 

 mits me to refer to it. This egg offers especially clear evidence of the persistence 

 of the sperm-centrosomes in the cleavage, because there are no asters associated 

 with the maturation spindles, and the sperm-asters are perfectly distinct, and per- 

 sistent from the time of entrance of the spermatozoon. 



