158 Edward Hindle 



arrange themselves about the equator of this amphiaster to form a 

 nuclear spindle. In the chemically fertilized eggs a nuclear spindle 

 arises in a similar way and the chromatin assumes the form of a 

 spireme preparatory to breaking up into chromosomes, but, instead 

 of 36, only 18 of these latter bodies appear. The subsequent changes 

 are identical in both kinds of eggs. The chromosomes split longitu- 

 dinally and each half moves along the spindle fibres towards its 

 respective pole. As they approach the poles the chromosomes swell 

 up and eventually fuse together to form a single nucleus in the 

 region occupied by each of the diffuse centrosomes. Meanwhile a 

 cell wall develops between the two nuclei dividing the cytoplasm 

 into two, and finally the spindle fibres disappear. The succeeding 

 processes of development, both internal and external, are similar 

 in both naturally and chemically fertilized eggs, with the exception 

 that at each succeeding division only 18 chromosomes appear in the 

 latter instead of the normal number, 36. 



Summary of Results. 



A cytological study of the changes occurring in the eggs of 

 Strongylocentrotus purpuratus after chemical fertilization by means 

 of LOEB'S improved method. 



(A.) Effect of treatment with butyric acid. 



1) The first change is the starting of a process of cytolysis 

 resulting in the formation of a fertilization membrane. 



2) This is accompanied by an alteration in the appearance of 

 the nucleolus, which, from being a dense mass of chromatic substance, 

 changes to a lightly-staining body of somewhat indefinite shape. 



3) A dissolution of the cytoplasmic granules in the immediate 

 neighbourhood of the nucleus results in the appearance of a clear 

 perinuclear zone. Probably as a result of currents flowing centri- 

 petally from the cytoplasm towards the nucleus, radiations appear 

 in and around this zone. 



4) The appearance of these radiations is succeeded by a period 

 of growth during which there is an increase in the size of the 

 nucleus. 



5) In eggs that are developing at ordinary temperatures a large 

 monaster is now developed, it's rays centering in the nucleus. The 

 nuclear membrane disappears and the chromatin breaks up into 18 

 chromosomes. These may undergo division and be drawn out of the 



