A Cytological Study of Artificial Parthenogenesis etc. 155 



and the succeeding stages in the division of the cell are identical 

 with those described above. 



The cytoplasmic granules are gradually used up as development 

 proceeds and disappear before the gastrula stage is reached. 



As the cells become smaller at each division, it becomes in- 

 creasingly more difficult to count the number of chromosomes with 

 any degree of accuracy, owing to their smaller size and also to the 

 smallness of the cells, which prevents the chromosomes separating 

 distinctly. But it is possible to obtain a close approximation of their 

 number as late as the free swimming blastula stage, in which there 

 are at least 512 cells, and we have invariably found it to be in the 

 neighbourhood of 18 (Fig. 11). The reduced number of chromosomes, 

 therefore, may reasonably be supposed to persist throughout the 

 further stages of development, for a multiplication of their number 

 has never yet been observed in any dividing cells. 



In addition to the normal processes of development described 

 above, which occur in the majority of the eggs, abnormal pheno- 

 mena are always found in some of them. The reason for this 

 is evident, for it is obviously impossible to secure for all the eggs 

 exactly the same supply of oxygen and hence the effect of the 

 hypertonic solution cannot be the same for all the eggs taken out 

 at one time. 



The most commonly occurring abnormality is the development 

 of a varying number of asters in the cytoplasm quite independently 

 of the cleavage aster, which arises from the nuclear region. These 

 cytasters develop in eggs that have been over-exposed to the action 

 of the hypertonic solution and have not been observed after treatment 

 with butyric acid alone. 



Each cytaster consists of a number of fibres radiating from a 

 more or less indistinct central granule into the surrounding cytoplasm 

 (Fig. 15). As a rule they do not attain much development but remain 

 scattered around the periphery of the cell and disappear before the 

 completion of the first division. In certain cases, however, they may 

 become so well developed as to interfere with the normal division 

 of the egg, the rays of cytasters in the region of the nucleus be- 

 coming attached to the chromosomes and thus producing multipolar 

 mitoses. In these cases the chromosomes are divided irregularly 

 between the two poles of the cleavage amphiaster and the centres 

 of as many cytasters as form a connection with the nucleus. 



A cell wall may develop between any two centres of attraction 



