A Cytological Study of Artificial Parthenogenesis etc. 159 



nuclear area along the rays, in which case, they appear scattered 

 through the cytoplasm. Such eggs never divide but simply disinte- 

 grate by a process of cytolysis. 



6) In eggs that are developing at a low temperature (2 5 C.) 

 the first few divisions may be accomplished, in which case the period 

 of nuclear growth is succeeded by exactly the same changes as 

 those occurring in eggs that have been subsequently treated with 

 a hypertonic solution, described below. 



7) Cytasters are not developed. 



(B.) Effect of treatment with butyric acid followed by treatment 

 with hypertonic solution. 



8) The interval (15 20 mins.) between treatment with butyric 

 acid and with hypertonic salt solution is characterized by the 

 membrane formation and an alteration in the staining properties of 

 the nucleolus. These changes are accompanied by the appearance 

 of a clear perinuclear zone as described above. 



9) During the treatment with hypertonic salt solution there is 

 a slight increase in the size of the nucleus and the clear zone almost 

 disappears. 



10) After transference of the eggs back into normal sea water 

 the perinuclear zone reappears, and is followed by further growth 

 of the nucleus. 



11) A typical cleavage aster develops in the nuclear region, its 

 rays being focussed in two areas, one at each pole, that probably 

 represent diffuse centrosomes. 



12) A varying number of asters, quite independent of the 

 cleavage aster sometimes appear in the cytoplasm. If excessively 

 developed they interfere with normal division of the cell and multi- 

 polar spindles are often formed. In eggs that have not been exposed 

 too long to the action of the hypertonic solution, the cytasters dis- 

 appear before the completion of the first division. 



13) The chromatin assumes the form of a spireme and then 

 breaks up into 18 chromosomes, which is exactly half the number 

 occurring in normally fertilized eggs. They arrange themselves in 

 the cleavage aster, and divide in the usual way forming two daughter 

 nuclei each with 18 chromosomes. This process is accompanied by 

 division of the cytoplasm. 



14) The reduced number of chromosomes, 18, persists in the 

 cells of the parthenogenetic larvae at least as far as the blastula, 



