130 EXTERNAL FACTORS III. 8 



Should the pronuclei unite which is only possible before these 

 pseudasters have been developed, if the eggs have been sub- 

 jected to the action of the poison immediately (one minute) after 

 fertilization the conjugation nucleus itself becomes the focus 

 of a similar system. In eggs poisoned after a longer interval 

 (fifteen minutes) the male and female pseudasters may them- 

 selves unite. 



The nucleus or nuclei divide irregularly, the chromosomes 

 passing in unequal numbers to the poles of the figure. The 

 several pseudasters and isolated asters, with which nuclei may 

 possibly become secondarily associated, may be united by clear 

 streaks of protoplasm, thus giving rise to a dendritic figure. 

 Simultaneous and unequal division of the whole ovum follows. 



Should the spermaster have already been developed fifteen 

 minutes after insemination it degenerates. The subsequent 

 changes comprise the formation of multipolar figures and 

 irregular cell-division. 



In later stages when fertilization has been completed and 

 segmentation is about to begin the ova are almost or quite 

 indifferent to nicotine, strychnine, and morphine; but chloral 

 (0-5 %) destroys the asters which are already in existence and 

 brings about a reconstitution of the combination nucleus with 

 subsequent formation of a tetraster and quadruple division. In 

 future mitoses, however, the spindles are bipolar. Cocaine and 

 quinine (-05 %) (Fig. 68,/-//) have the same effect. 



The importance of these experiments does not require to be 

 emphasized. Not only do they throw a valuable light on the 

 possible causes of those pathological mitoses that occur in 

 malignant growths, they also contribute very greatly to the 

 understanding of the normal processes of fertilization and 

 karyokinesis. 



Thus from the failure of the asters to appear in eggs treated 

 with chloral before fertilization the brothers Hertwig argue that 

 the contractility of the cytoplasm is diminished by this substance, 

 and from the failure of the pronuclei to unite in eggs which 

 have been immersed in the solution shortly after fertilization 

 they suggest that it is the contractility of the ovum which 

 normally brings about the union of the pronuclei. Since, 



