STUDY OF CELL MECHANICS 473 



and female pronuclei. It is also interesting to note that the 

 clear fluid area accompanies the male pronucleus up to the time 

 of fusion, but after this it disappears. The swelling following 

 the union of the pronuclei goes on rapidly and the nucleus 

 becomes very large. At the same time, the chromatin collects 

 into definite bodies and just before the nuclear wall disappears 

 the chromosomes show very distinctly, lying on the linin net- 

 work. As the nuclear wall is dissolving one occasionally sees 

 a centrosome, (fig. 27), which appears to lie on the nuclear wall, 

 and the fibers from this center appear continuous with the 

 linin net-work in which the chromosomes lie. The division of 

 the centrosome seems to already have taken place, for with the 

 disappearance of the nuclear wall, a spindle is formed within 

 the nuclear area (fig. 28). The whole nuclear cavity is filled 

 with exceedingly fine parallel fibers and the chromosomes lie 

 scattered in an irregular equatorial plate. Although I have 

 studied these preparations with the best lenses I have been un- 

 able to detect a centrosome at either end of the nuclear area. 

 At this time there are absolutely no cytoplasmic radiations 

 to be found in the eggs of my preparations. Following this 

 an irregular sort of division of the chromosomes seems to take 

 place (fig. 30) and chromosome vesicles form at the two sides 

 of the nuclear area. 



In those cases where the pronuclei did not fuse, we find that 

 the nuclear walls disappear at the same time. The chromo- 

 somes lie on a network which in some cases has a parallel struc- 

 ture, like that of a spindle (fig. 29). An irregular division of 

 the chromosomes takes place here too and afterwards the chro- 

 mosome vesicles are formed (fig. 31). 



At the time when the nuclear vesicles are fusing into one 

 or more nuclei, we find the movement of the protoplasm be- 

 ginning. There is the greatest variation in the appearance of 

 the eggs but they all agree in this that the cleavage furrow al- 

 ways cuts through the place occupied by the nucleus (figs. 32 

 and 33). 



The point which I wish to emphasize is this, that at no time 

 during the whole history of these eggs, is there any trace of 



