270 GENETICS 



that have begun to develop, and are nearly ready to divide, 

 are fertilized, the larvae produced are those of the mother 

 species only. But if fertilization occurs before the egg nu- 

 cleus has enlarged in preparation for division, the larvae 

 produced are intermediate in character between those of 

 the two parents. 



Cytologlcal examination showed that In the cases in 

 which the egg nucleus had begun the developmental proc- 

 esses before fertilization occurred, the sperm chromosomes 

 lag behind those from the egg nucleus. They do not succeed 

 in uniting with egg chromosomes into a single nucleus. In 

 cell division the sperm chromosomes are seen lying to one 

 side, in a less advanced condition than the egg chromosomes 

 (figure 54). They never catch up in the developmental proc- 

 esses. They are irregularly distributed to the various cells 

 produced; they become abnormal and finally some or all of 

 them die. In such cases the larvae produced are either com- 

 pletely maternal in their characteristics (figure 53, D), or 

 with a slight admixture of paternal characteristics. The 

 different results seemingly depend on how far behind the 

 paternal chromosomes are, and whether they are finally 

 completely eliminated. 



In case however fertilization occurs before the egg nu- 

 cleus has begun to enlarge for development, the paternal 

 and maternal chromosomes take equal parts in develop- 

 ment, and the larvae produced are therefore Intermediate. 



Similar conditions have been shown by Tennent and 

 others to prevail in crosses between a considerable number 

 of different species of sea urchins. A case of perhaps special 

 Interest has been described by Baltzer.^ When the ova of a 

 species of Paracentrotus are fertilized by sperm of a species 

 of Sphaerechlnus, the chromosomes from the sperm are 

 almost entirely eliminated (apparently four out of eighteen 

 persist). The larvae produced are almost completely ma- 

 ternal In character. On the other hand, in the reverse cross. 



