EFFECTS OF MIXING DIVERSE ORGANISMS 273 



Often the chromosomes of two species differ in number 

 and size. Frequently it is found that In such cases they do 

 not work well together in development. The cross-fertilized 

 egg begins to develop, but soon development ceases and the 

 embryo dies. In Moenkhaus' work on crossing two genera 

 of fish, Fundulus and Menldla,^ It was found that the chro- 

 mosomes of the two differ greatly (figure 16, in Chapter 

 II). The egg containing the two kinds of chromosomes 

 commences development. But soon development takes an 

 abnormal course and after a time ceases. Many cases of 

 this type have been fully studied; they are common. 



2. Incompatibility of Paternal and Maternal Chromosomes 

 in the Formation of the Germ Cells of Hybrids 



In the cases thus far described, the chromosomes of the 

 father show themselves Incompatible with the chromosomes 

 or cytoplasm of the mother, so that either the paternal 

 chromosomes are eliminated, or development becomes ab- 

 normal and ceases. 



In another class of hybrids the Incompatibility of pa- 

 ternal and maternal chromosomes does not show itself un- 

 til the hybrids form germ cells for the next generation. 

 The individual hybrid produced by germ cells from differ- 

 ent species develops well, may even show hybrid vigor, 

 becomes adult. The two kinds of chromosomes work to- 

 gether well in the development of the individual. 



But when in this hybrid individual the period arrives for 

 the two kinds of chromosomes to conjugate in forming 

 germ cells, the incompatibility shows Itself. In the intimate 

 union of chromosomes at conjugation, the two types of 

 chromosomes injure or destroy one another, so that no 

 germ cells are found. The hybrid individuals are therefore 

 sterile, or partly sterile. 



Sterility in hybrids is very common. A typical example 



