No. 3.] THALASSEMA AND ZIRPHAEA. 623 



b. In Cleavage. 



In mode of entrance, in appearance and general behavior, 

 the sperm-head of Zirphaea perfectly resembles that of Tha- 

 lassema. By second polar telophase the vesicular condition is 

 assumed and the volume enormously increased (PL XXXIV, 

 Fig- 53)- The intermediate stages in this transformation were 

 not observed. The vesicular sperm-nucleus is completely filled 

 by a rather close chromatic reticulum feebly staining with iron 

 haematoxylin, but deeply with Congo red. It is somewhat 

 pointed in outline, with the sharp end directed toward the 

 centrosome. The egg-nucleus, which soon becomes consti- 

 tuted by the fusion of the vesicles left within the egg after the 

 second polar division, is quite similar in all respects to the 

 sperm-nucleus. 



With continued approach, the germ-nuclei increase in size 

 and become perfectly smooth and spherical in outline. Their 

 size is, to all appearance, equal. Meanwhile the chromatin 

 shows a marked increase in its staining power, and just previ- 

 ous to copulation it stains as deeply as the chromosomes {cf. 

 Klinckowstrom, '97, for similar behavior of chromatin in Pros- 

 tkeceraeus). The chromatin now e.xhibits a disposition to arrange 

 in dark-staining strands, the initial stage in spireme-formation. 

 The nuclei have now approached and partially fused to form a 

 bi-lobed segmentation-nucleus. The ingrowing spindle-fibers 

 indicate the commencement of spindle-formation. 



In the next stage obtained, the spindle is completed and con- 

 tains a dense equatorial plate of rod-like chromosomes. Both 

 equatorial (PI. XXXIV, Fig. 61) and longitudinal sections show 

 these to be arranged in two separate groups that are evidently 

 maternal and paternal, respectively. This agrees with the 

 facts observed by Boveri ('90) in Pterotrachea; Hacker ('92) ; 

 Riickert ('93) in Cyclops; Herla ('93) in Ascaris ; Sobotta 

 ('95) in the mouse ; and others that have described a similar 

 independence of the egg- and sperm-chromosomes. By careful 

 orientation of the spindle with reference to the polar bodies, it 

 is seen that the line joining the centers of these chromosomal 



