400 H. H. NEWMAN 



Fundulus has two well-defined types of chromatophore : a type 

 of melanophore (black chromatophore) characterized by large 

 squarish body and few short branches, showing also a tendency 

 to fuse into syncytia, and a red or red-brown chromatophore 

 which in its definitive or fully expanded condition is very intri- 

 cately branched. The black type, if unhealthy, may remain 

 small, and give out a few slender branches, but is never a very 

 intricately branched cell in pure Fundulus embryos. The red 

 chromatophores also may be relatively simple or unbranched if 

 the embryo is pathological or retarded. 



The mackerel larvae also have two types of chromatophore 

 equally characteristic of the species and quite distinct from those 

 of Fundulus. One type is a melanophore which is characterized 

 by a small core or body and very slender anastomosing branches. 

 They never exhibit the tendency to fuse into syncytia. The 

 second type is an oKve-green chromatophore that occurs in the 

 larvae both on body and yolk sac. Usually there are two large 

 green cells just back of the eyes, two more a short distance 

 behind the otic vesicles, and two or three adjoining the Kupfer's 

 vesicle on the yolk sac. There are no red cells in mackerel 

 nor any green cells in Fundulus. So in these two opposed char- 

 acters there is a sharp contrast, and the finding of a red chromat- 

 ophore in a mackerel egg-hybrid or a green chromatophore in a 

 Fundulus egg-hybrid could not be interpreted as within the 

 range of variabiUty of the maternal species or as a pathological 

 occurrence. A fair-minded critic will admit that the only rea- 

 sonable explanation is that the factors for the foreign type of cell 

 have been introduced by the foreign sperm. 



Methods and general results of the hybridization experiments 



During the latter half of June, when both Fundulus and mack- 

 erel are at the height of the breeding season, is the best time to 

 make the crosses. Results of experiments made later in the 

 season, although to all appearances development is more fre- 

 quently normal, may be quite different. 



