26 HISTORY OF THE EGG FEOM FERTILIZATION TO CLEAVAGE. 



continuous, and the astral rays, although more strongly expressed on 

 the side of the male elements, appear to surround both nuclei. 



e. Conjugation of the Pronuclei. — From twenty to thirty minutes after 

 fertilization, the nuclear bodies are found in various stages of conjugation 

 (Figs. 21-30,' PI. XXIII.). The chromatic bodies present themselves as 

 homogeneous vesicles, spherical or hemispherical in shape, and measuring 

 each about 10 /j- in diameter. The radiation about the periplast is not 

 very distinct in our preparations of this age. 



The plane of junction of the two nuclear bodies may be vertical or 

 horizontal, or present any degree of inclination between these two ex- 

 tremes. Whatever the position of this plane at the start, we invariably 

 find it horizontal before the apparent coalescence takes place. We say 

 apparent, because we have seen indications of- incomplete coalescence even 

 after the amphiastral figure of cleavage appears. Everything that we 

 have seen indicates that the position of equilibrium ultimately reached 

 by the pronuclei is a constant one, the female pronucleus forming the 

 upper and the male the lower hemisphere of the cleavage nucleus. 



The rotation of the two nuclei during conjugation, to whatever extent 

 is required in order to bring the plane of junction into its normal hori- 

 zontal position, is evidently an adjustment of the same nature as that 

 first described by Auerbach 23 in the egg of Ascaris nigrovenosa, 



£. The Movements of the Pronuclei. — An attempt made by one of us 

 (No. 11) to analyze the movements of the pronuclei in different eggs led 

 to the recognition of three facts as furnishing the best evidence of attraction 

 between these bodies : — 



1. The curved path of the male pronucleus in the amphibian egg. 



2. The meeting of the pronuclei before reaching the centre of equilibrium. 



3. The centrifugal movement of the earlier pronucleus to meet the later 

 formed pronucleus. 



In the fish egg we have not found what would pass as incontrovertible 

 evidence of nuclear attraction, but it must be remembered that the con- 

 ditions here are not favorable for obtaining such evidence. We know 

 that either pronucleus is capable of performing its centripetal journey in- 

 dependently of the other, since it is sometimes the male and sometimes 

 the female pronucleus that normally reaches the centre of the active 

 cytoplasm first. But what ground have we for asserting that the move- 



23 Leopold Auerbach. Orgauologische Studicu, Breslau, 1S7L P- 212. 



