HISTORY OF THE EGG FROM FERTILIZATION TO CLEAVAGE. 33 



C. History of the Cleavage Nucleus. 



1. The Amphiaster. — About thirty-five or forty minutes after fertil- 

 ization, the time varying according to the temperature, the first amphiaster 

 of cleavage appears (Figs. 31-34, PI. XXIII.). The cleavage nucleus, as 

 we have seen, is formed by the union of equal pronuclei, is spherical at, 

 the end of conjugation, and consists of an upper female, and a lower 

 male hemisphere. The line of demarcation is rapidly effaced in the 

 living egg; by this we mean only that the line becomes invisible under 

 the conditions of examination. Were it possible to use higher powers 

 under favorable light, it is not improbable that we might see the dividing 

 line for at least some live minutes after the spherical form is reached. 

 In sections the boundary line is sometimes recognizable forty minutes 

 after fertilization, after the appearance of the amphiaster (Fig. 32). 



The spherical form is scarcely reached before the nucleus begins to 

 elongate at right angles to the axis of the egg, passing into an oval 

 form, in which its longer horizontal axis measures 15-20 p, its vertical 

 axis from 10-12 /x. These dimensions are somewhat smaller than we 

 should expect, from the measurement of the spherical nucleus in the 

 living egg; but we made only one such measurement, and. judging from 

 what we find in sections, there must be considerable variation. 



Simultaneously with this elongation of the nucleus begins the flatten- 

 ing of the inner face of the blastodisc, which, as we have before re- 

 marked, is the first outward premonition of the approach of cleavage. 

 It is at this time, just a moment before the outline of the nucleus fades 

 from sight in the living egg, that we have obtained the earliest phases of 

 the amphiaster. Figures 31 and 34 represent the earliest appearances, 

 but Figures 32 and 33 are but little more advanced. In Figure 31 the 

 outline of the nucleus is apparently complete, but is less distinct at the 

 poles than elsewhere. In Figure 34 the whole outline is less sharp, and 

 it is partially effaced at the poles, close to which are seen the minute asters. 

 The chromatic elements are unevenly elongated, the general trend being 

 parallel to the long axis of the nucleus, which produces an indistinctly 

 striated appearance. No definite arrangement could be made out, and 

 no indication of an equatorial plate was discovered. The more fluid 

 constituents of the nucleus appear homogeneous, and evenly darkened 

 with the osmic acid stain. The clear area around the conjugating nuclei 



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