Cardiff : Synapsis and reduction 289 



some passes undivided to one pole of the nucleus in the first division 

 and divides in the homotypic division, one half of the microspores 

 should contain each one more chromosome than the other half, 

 but I have no direct evidence to show that such is the case. Wil- 

 son in his recent investigations upon chromosomes seems to have 

 proved beyond a doubt that these chromosomes of unusual 

 behavior are in some way connected with sex-determination. 



He finds in a number of Hemiptera that the oogonial cells 

 contain one more chromosome than the spermatogonial cells, the 

 latter having an odd number and the extra chromosome (hetero- 

 tropic) goes over undivided in the first maturation division. Thus 

 half of the sperms have the same number of chromosomes as the 

 unfertilized eggs, while the other half have one less than this 

 number. 



In the spermatogonia of other forms he finds that in one of 

 the bivalent chromosomes the univalent parts are of unequal size. 

 As a result it divides unequally and one half the sperms have one 

 chromosome (idiochromosome) smaller than the other half. In 

 the oogonial division of this group all chromosomes divide 

 equally. 



In one form (Nezard) Wilson finds that all the heterotypic 

 chromosomes divide equally in both sexes. One pair of chromo- 

 somes, however, agrees in behavior with the idiochromosomes of 

 the previous group though they have not yet become differentiated 

 in size. Wilson's work gives the most tangible evidence yet 

 obtained upon the behavior of the individual chromosomes in the 

 germ cells and should the idiochromosomes or their homologues 

 prove to be of general occurrence a most important advance will 

 have been made in this field of cytology. 



Even if the heterotropic chromosome of Salomonia is of the 

 same significance as in the Hemiptera studied by Wilson, its be- 

 havior and later history will not be so simple on account of the 

 fact that Salomonia is a bisporangiate plant. It may, however, 

 be related to prepotency of microspores. 



Ginkgo biloba L. 



While the microsporangia of Ginkgo are very abundant and 

 accessible and preparations from them are made with little diffi- 



