THE MODIFICATIONS OF THE CHROMOSOME CYCLE 63 



from a cross fertilization are entirely paternal in character (e.g. Nico- 

 tiana). In these "hybrids" it must be assumed that the reduced egg 

 cell has been fertilized by the male gamete, and that the female nucleus 

 has subsequently degenerated. Similar phenomena have been produced 

 artificially in animals (sea urchins, newts), in which it has been possible 

 to fertilize enucleate egg fragments with sperm either of the same 

 species (merogony) or of a different species (bastard merogony). We 

 shall return to a consideration of these experiments in connection 

 with the relations of the nucleus and cytoplasm during development 

 (Chap. 6). 



3. The Production of Unreduced Gametes 



The production of unreduced gametes, regularly in diploid partheno- 

 genesis, and occasionally in sexual organisms where they lead to the 

 formation of polyploids, may be due to (i) a failure of pairing, or 

 (2) a failure of the mechanism for separating the chromatids. 



(i) A failure of pairing in regularly diploid-parthenogenetic organ- 

 isms may be due to a genetically controlled failure of the conditions 

 which normally cause meiosis, in particular to a failure of the pre- 

 cocious chromosome contraction. In other cases it is due to too great a 

 dissimilarity between the chromosomes. This may occur in true 

 haploids of non-polyploid species, e.g. Datura^ or in hybrids, e.g. 

 Raphanus-Brassica. In such cases there is no pairing and therefore no 

 chiasma formation. The chromosomes are usually scattered over the 

 spindle in the first division and pass at random to the two poles; 

 lagging chromosomes may form subsidiary nuclei. Unreduced gametes 

 are only formed if these groups of chromosomes subsequently unite, 

 forming a sort of restitution nucleus. Some chromosomes may pass on 

 to the metaphase plate and divide at the first division; if many of them 

 do so, the second division is often suppressed, but otherwise the 

 chromosomes which fail to divide at the first division divide at the 

 second. The process is clearly a very irregular one, and will give at best 

 a very few haploid gametes from a haploid organism or diploid ones 

 from a diploid hybrid. 



Meiosis in haploid cells leads to a regular result only in males pro- 

 duced by haploid parthenogenesis. Thus, in male bees no chromosome 

 pairing takes place but all the chromosomes pass to one pole and the 

 division merely results in the pinching off of an enucleate bud of 

 cytoplasm. In rotifers and coccids the formation of unreduced gametes 

 is rather less regularly ensured by the failure of one division. 



