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THE AMERICAN MUSEUM JOURNAL 



seen as small particles in the form of loops or bands, more or less 

 compactly arranged, and of a definite numbsr in any given species. 

 To these the name " chromosomes " has been given. The division 

 of a nucleus seems to consist mainly in a careful separation 

 of the chromosomes into two equal parts. 



There is also present in connection with each nucleus a small 

 body which seems to be the center of all nuclear changes, the 

 *'centrosome." Whenever any activity of the nucleus such as 

 a division takes place, the centrosome is in evidence. 



Centrosomes are to be observed in both of the pronuclei of the 

 undivided Q.g%, and radiations apparently extend from them 

 to each separate chromosome. The arrangement of the chro- 

 matin now becomes looser, and the chromosomes are more 

 widely separated. The centrosomes come to lie in diametrically 



Fig. 



nuclear boundaries next disappear, the chromosomes become 

 opposite positions with the two pronuclei between them. The 



MODEL 2 



First cleavage. Separation of chromosomes. Elongation and constriction 

 of the egg preceding its complete division into two cells. 



