William J. Moenkhaus 55 



The rhythm of cleavage is that of the egg species. A spermatozoan 

 from a species that normally has a different rate of cleavage cannot 

 modify the rate of the hybrid egg. 



Hybrid eggs may develop more slowly than normal eggs. This usu- 

 ally does not appear imtil later stages. As development proceeds the 

 difference in rate grows increasingly great. 



Dispermic eggs fall at once into four cells of the normal size and 

 arrangement. This is followed by a normal 8, 16, 32, etc. cell stage. 



The dispermic eggs of the Fundulus hybrid may develop to a late 

 cleavage stage but never form a germ ring or embryonic shield. 



The chromosomes of the two parent species, Fundulus heteroclitus 

 and Menidia notata, are morphologically distinguishable, the rods of 

 the former being long and straight in form, those of the latter, shorter, 

 and commonly slightly curved. These retain their characteristic form 

 when introduced into a strange egg through hj^bridization. 



During the development of the hybrids they retain their individuality. 

 During the first two cleavages each kind remains grouped and bilaterally 

 distributed on the spindle. During the resting stage of the four-cell 

 stage the chromatin becomes more or less mingled, so that when the 

 third cleavage spindles are formed the grouping and the bilateral dis- 

 tribution of the chromatin has largely disappeared. During the follow- 

 ing resting period the mingling has gone further, so that a complete 

 grouping of the two parental chromosomes occurs very rarely in the 

 following division. During the subsequent cleavages to a late cleavage 

 only the mingled condition was observed. 



This mingling of the chromosomes does not destroy their individuality 

 for in stages of division favorable to bringing out the form of the chromo- 

 somes both kinds can be readily seen. 



In these hybrids any nuclear conditions which would indicate that the 

 chromatin is bilaterally arranged does not indicate any bilateral distri- 

 bution of the two paternal chromatins in those nuclei. 



The mingled condition of the maternal and paternal chromosomes in 

 all but the very early stages of cleavages in these hybrids makes the bilat- 

 eral distribution in the other forms described — Ascaris, Cyclops, Crepi- 

 dula and Pinus — an open question. 



The conditions obtaining in these hybrids are considered among the 

 strongest evidences in support of Boveri's hypothesis that the individual 

 chromosomes persist and do not mix in the resting stages of the nuclei- 



