62 PROCEEDINGS OF THE AMERICAN ACADEMY 



Nuclei. — We have obtained a very nearly complete history of the 

 nuclei as far as the G i-cell stage ; but we have obtained this from 

 mounted preparations of the blastodisc, which, on account of their 

 thickness, do not admit of examination with very high powers. To 

 this fact must be attributed our failure to analyze the various con- 

 ditions assumed by the chromatic elements, with such detail and 

 completeness as have been attained on more favorable objects by 

 Flemming, Strasburger, and Van Beneden. 



Contrary to what is seen in most of the figures of Flemming and 

 Strasburger, the chromatine figures form only a minimal portion of the 

 araphiasters. The first cleavage-nucleus is, in the living ovum, only 

 .02 mm. in diameter. The amphiastral figure is always present before 

 the outline of the nucleus is lost. The nucleus presents at first nearly 

 a spherical form, then an oval form just before vanishing. Prepara- 

 tions of the nucleus in this oval or elliptical form usually show signs of 

 division in the equatorial plane, which indicates that the chromatine 

 fibres have already arranged themselves in two groups. At this time 

 the preclcavage plane is already established, and is coincident with the 

 equatorial plane of the nucleus. About this time tlie nucleus becomes 

 invisible in the living condition. The two nuclear plates move towards 

 the opposite poles of the amphiaster, assuming a rounded contour just 

 hefo7-e, or at the moment of, reaching the edge of the polar areas. 

 The polar areas, the centres of the asters, are irregular in outline, 

 often amccboid in form, and stain very little or not at all. The 

 spindle-fibres are very feeble, and scarcely distinguishable from the 

 astral lines. The chromatic elements eventually reach the centres of 

 the polar areas, but not until about the time these elongate to form a 

 new generation of amphiasters. The astral lines are not lines in the 

 strict sense of the word ; for they appear to be made up of linear and 

 somewhat fusiform elements, each having a radial direction, and thus 

 producing the impression of radial lines. The spindle-fibres, in many 

 CMses, show a very feeble staining. The size of the achromatic polar 

 areas forbids the idea that anything more than a small part of their 

 substance is derived from the nucleus. Although the achromatic por- 

 tion of the amphiaster appears to take the lead in tlic process of 

 division, we are by no means certain that the chromatic elements do 

 not play an active part. The extremely interesting investigations of 

 Van Beneden ^^ on the distribution of the chromatine in different 



^ E. van Bencilen. " Rccliorclics sur In Maturation de I'OEuf et la Fe'conda« 

 tion." Arch, de Biol., IV., Tarts 2 and 3, lSb4. 



