William J. Moenkhaus 



35 



tions on a great many other equally distant crosses which were made 

 among fishes. 



6. Rhythm of Cleavage. — In .general, as already pointed out by Born, 

 83, the hybrid egg develops slower than the normal. I have observed 

 this repeatedly in fishes. In most of the crosses that have come under 

 my observation, however, the difference in the rate is very slight and 

 cannot in most cases be detected in the early cleavage stages. In the 

 following table is given a comparison of a lot of hybrid eggs with a 

 lot of normals. The eggs were taken from the same mother at the same 

 time, fertilized at the same moment and kept under exactly similar con- 

 ditions. The observations were made at the same time on both batches of 

 eggs and the stage at which each was found was indicated as accurately 

 as possible. 



1 Eggs fertilized at 7 P. M., June 26. 



From the table it will appear that the retardation in the development 

 does not appear until the close of cleavage. If the development of the 

 hybrid is slower than the normal during the first four or five cleavages it 

 is so slight that it cannot be detected. From this time until the time 

 of gastrulation the hybrids fall considerably behind. From the time 

 of gastrulation on they fall increasingly more behind the normals. It is 

 probable that the slowing-up process does not take place at the same 

 rate ■ but that it becomes increasingly rapid as development proceeds. 

 In the reciprocal cross, if compared with the normal eggs of Menidia, 

 the same conditions obtain. The normal Menidia eggs cleave a little 



