No. 3.] THALASSEMA AND ZIRPHAEA. 589 



be seen later, when discussing the rotation of the first polar 

 amphiaster. 



The nucleolus was first noticed upon the breaking up of the 

 spireme. Its intense blackness and rough and irregular outline 

 give the impression of an irregular remnant of the spireme, and 

 with Auerbach's fluid it can often be made to take a purer green 

 than the chromosomes. The persistent nucleolus undergoes 

 no change until the early prophase of the first polar mitosis. If 

 it then happen to be near either of the asters, but not other- 

 wise, it assumes an elongate ovoid outline, with the narrow end 

 directed toward the center of the aster (PI. XXXI, Fig. 1 1). 

 Two portions are now distinguishable, a larger and more darkly 

 staining area involving the pointed end, and a smaller cap-like 

 portion at the blunt extremity. The two portions are not, 

 however, to be compared to the Haupt and Nebentheil of the 

 lamellibranch nucleoli, since they stain similarly and are not 

 constant features. The nucleolus subsequently undergoes reab- 

 sorption along with the remnants of the nuclear skein (PI. XXXI, 

 Fig. 10). 



II. General History of the Achromatic Structures. 



The egg of Thalassema is in many respects exceptionally 

 favorable for cytological study. It is large (70-80/a diameter) 

 and transparent, and the yolk is distributed in the form of large 

 deutoplasm-spheres in the meshes of an extremely coarse cyto- 

 plasmic reticulum (PL XXXI, Figs. 10-12). The yolk distri- 

 bution is, furthermore, such as to leave a relatively unobstructed 

 field for the play of the mitotic phenomena, and in consequence 

 the achromatic structures reach a powerful development, and 

 comparatively little difficulty is experienced in following the 

 history of the centrosome. 



a. The First Polar Division. 



Origin of the Amphiaster. — ■ The earliest stage that yields 

 unequivocal traces of centrosomes is found among preparations 

 of eggs fixed three minutes after fertilization. Their presence 

 is then revealed by two excessively minute asters situated close 



