504 MARGARET MORRIS 



They reached a stage in which the embrj^o was fairly well formed 

 and the germ-ring had gone two-thirds of the way around the 

 egg. 



The cytology of hybrids has attracted considerable attention in 

 recent years. In 1904 W. J. Moenkhaus made a study of the hy- 

 brids between Fundulus heteroclitus and ]\Ienidia notata, which is 

 of particular interest in the present connection. In the F. 9 X ^I. cf 

 cross, Moenkhaus finds that about 50 per cent of the eggs are 

 dispermic, while in the reciprocal cross there are only a few which 

 are poly- or dispermic. He did not succeed in keeping the hybrid 

 larvae alive more than two or three days, and therefore made 

 no study of their heredity. The most important part of the 

 paper is the study of the germ-nuclei and the chromatin in the 

 early cleavages. ]Moenkhaus says that the pronuclei fuse com- 

 pletely before the first cleavage, but he gives no figures to demon- 

 strate that fusion. He passes at once from a figure which shows 

 the pronuclei closely apposed — as he says, "about to fuse" to 

 the metaphase of the first cleavage spindle. In this stage and 

 in the anaphase, he finds two morphologically distinct kinds of 

 chromosomes, each of which can be traced to one parent of the 

 cross. His figures show the two types very plainly, and he traces 

 these types as far as the late cleavage stage. At first the chro- 

 mosomes are bilaterally grouped, that is, the paternal and mater- 

 nal chromosomes are not mixed on the spindle, but in the third 

 cleavage he finds this grouping destroyed, and as cleavage 

 progresses the two types are completely mingled, though still 

 persisting. 



The work of D. H. Tennent COS) on the eggs of Toxopneustes 

 and Arbacia fertilized with Aloira sperm shows somewhat similar 

 results. He finds that the pronuclei fuse completely; })ut in 

 the Arb. 9 X M. d" cross in which the two types of chromosomes 

 are morphologically distinct, they are mingled, not grouped on 

 the first cleavage spindle. 



The paper of Jacques Loeb ('12) on heredity in heterogeneous 

 hybrids forms a complement to the work of Aloenkhaus, as it 

 discusses the inheritance of maternal characters in hybrids be- 

 tween Fundulus and Menidia, Ctenolabrus, and Stenotomus 



